Adding data to an existing table is an important part of maintaining a current and complete GIS. There are several ways for easily adding data to tables in ArcGIS.
Using 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.
The Append tool
Use this tool to add new features or other data from multiple datasets into an existing dataset. This tool can append point, line, or polygon feature classes, tables, rasters, annotation feature classes, or dimensions feature classes into 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.
The Calculate Field tool
Calculates the values of a field for a feature class, feature layer, or raster catalog.
The Calculate Field tool is great for updating either existing fields or newly created fields. You can calculate numbers, text, or date values into a field. Using code blocks, you can write scripts to perform advanced calculations.
The Add Join tool
The Add Join tool will append 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 can be found in both tables. The name of the field does not have to be the same, but the data type has to be the same; you join numbers to numbers, strings to strings, and so on. You can perform a join with either the Join Data dialog box, accessed by right-clicking a layer in ArcMap, or the Add Join tool.
When you create a joined table, the fields that are appended from the join table are not permanently attached to the base table. You can remove a join to remove the appended fields.
Using joins to add data
You don't have to use geoprocessing to create a join. You can use the Join Data dialog box in ArcMap to create a join. Whether created using the Join Data dialog box or with a geoprocessing tool, the join will behave in the exact same way. Join fields are appended to the base table (or target table), and the appended fields can be used in field calculations or labeling, symbolizing, or querying the data.
Adding data by editing
Editing data to update existing attributes or to create new data is a process than can be done in ArcMap. You can edit attributes through the attribute table of a layer or table or by using the Attributes window.
The Attribute table
When you open an attribute table, the default view of the table is read-only. However, if you start an edit session, you can manually edit the attributes in the cells of the table. When you are editing in the attribute table, a blank row is added to the bottom of the table where you can add new data to the table.
To make automated edits in the attribute table, you can use the Field Calculator or Calculate Geometry tools to update the table. You can use these tools outside an edit session; however, you will not be able to undo your calculations unless you are in an edit session.
The Attributes window
The Attributes window allows you to view and edit attributes of features you have selected. You can open it by clicking the Attributes button 
on the Editor toolbar.