To change variable values in a table, first double click on the table you need to change from SAP ABAP debugger code window on the left side. Then it will display on the right side of the SAP ABAP debugger. You can’t change table variable values in this window. You need to double click on the table again on the right side of the SAP ABAP debugger and go to a detailed overview of the table in another tab.
In that you can change the table variable value by selecting the row and go to Table Services->Change Row Content.