To change the value of variables, first double click on the variable 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. Double click on the pencil icon just after the existing value of the variable. The value field will become white and the pencil icon will turn into a glasses icon.
Now you can change the existing value of the variable. After changing the value of the variable it is very important to exit from the edit mode by clicking enter on the keyboard and save the changes.