Adding data to an existing table and editing that data in the table are important parts of maintaining a current and complete GIS. There are several ways to add data to tables.
Copy and paste from another application
You can add data to an existing table by pasting values from other applications such as Microsoft Excel. Copy and paste is a recommended workflow to update and replace existing values with new information. If more rows are pasted than currently exist in the database table's row count, additional rows are automatically created.
Note:
If more columns are pasted than currently exist, additional columns are dropped.
Copy and paste from another ArcGIS AllSource table
You can add data to an existing table by pasting values copied from another table in either the same or a different ArcGIS AllSource project. Copy and paste is a recommended workflow to update and replace existing values with new information. If more rows are pasted than currently exist in the database table's row count, additional rows are automatically created. However, if more columns are pasted than currently exist, additional columns are dropped.
Use geoprocessing to add data
With geoprocessing, you can use tools to update existing fields, append records to a table permanently, or append fields to a table dynamically with a join.
Calculate Field tool
You can use the Calculate Field tool to update existing fields or newly created fields for a feature class, feature layer, or raster catalog. You can calculate numbers, text, or date values into a field. Using code blocks, you can write scripts to perform advanced calculations.
Add Join tool
The Add Join tool links the fields from the join table to a base table.
Typically, you'll join a table of data to a layer based on the value of a field that exists in both tables. The names of the fields do not need to be the same, but the data type does; you join numbers to numbers, strings to strings, and so on.
When you create a joined table, you can use the appended fields in field calculations or for labeling, symbolizing, or querying the data. The fields that are appended from the join table are not permanently attached to the base table. You can undo joins to remove the appended fields.
Join Field tool
The Join Field tool appends the contents from one table to another table based on a common field. You can optionally define which fields from the join table will be added to the input table.
When using this workflow, the fields are permanently added to your base table.
Add Field tool
The Add Field tool adds a field to your current table or the table of a feature class, feature layer, raster catalog, or raster with an attribute table. Use Calculate Field to populate newly added fields.
Append tool
Use the Append tool to add features or other data from multiple datasets to an existing dataset. This tool can append point, line, or polygon feature classes; tables; rasters; raster catalogs; annotation feature classes; or dimensions feature classes to an existing dataset of the same type. For example, you can append several tables to an existing table or several rasters to an existing raster dataset, but you cannot append a line feature class to a point feature class.
Calculate Geometry Attributes tool
The Calculate Geometry Attributes tool adds information to a feature's attribute fields representing the spatial or geometric characteristics and location of each feature, such as length or area and x-, y-, z-, and m-coordinates.
Add rows to a table
Feature class attribute tables support the addition of stand-alone rows. To do so, click the Click to add new row option from within the attribute table.
Some feature layers do not allow the addition of stand-alone rows because their features require geometry. These layer types do not include the Click to add new row option:
Control the appearance of this option on the Table tab of the project options.
Insert rows into a stand-alone table
You can insert rows into an active, editable stand-alone table. Click the Insert Rows button 
and provide the Number of rows value to add to the table. Click Create or press Enter. The new rows are added to the bottom of the table and selected, and the first newly added row has focus.
Note:
- You can add a maximum of 1,000 rows at one time.
- If a definition query is being used, the new rows may not appear.
Duplicate a row in a table
You can create a duplicate copy of the attributes of a feature or record. Right-click the row header and click Duplicate Row 
to create a copy of the chosen row at the bottom of the table. It is selected. The geometry of a feature is included when duplicating a row.
Use Fields view to create, modify, and delete fields
The Fields view is used to manage the fields associated with a table. In the Fields view, you can edit a table's fields and modify field properties, delete fields, or create fields. To open the Fields view, right-click a column heading in the table and click Fields 
. You can also click Add Field 
from the table view's built-in toolbar to open the Fields view to add a field.