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.
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
The Calculate Field tool can be used 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 name 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, the appended fields can be used 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. Joins can be undone 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 desired 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 new 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 new 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, several tables can be appended to an existing table, or several rasters can be appended to an existing raster dataset, but a line feature class cannot be appended to a point feature class.
Add Geometry Attributes tool
The Add Geometry Attribute tool adds new fields and populates them with spatial or geometric characteristics of each feature, such as length, area, or x,y,z coordinates.
Use the Attributes pane to perform updates
Another way to update existing feature attributes is to use the Attributes pane. A selection is required to perform the edits, and you work from within the map or scene, instead of the open table. On either the Map or Edit tab, in the Selection group, click Attributes.
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 will automatically be created.
Use Fields view to create, modify, and delete fields
Fields view is used to manage the fields associated with a table. In 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 directly open the Fields view to add a new field.