To change variable values, double click on the structure 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 structure variable values in this window. You need to double click on the structure again on the right side of the SAP ABAP debugger and go to a detailed overview of the structure in another tab.
In that you can change the structure variable value by clicking the pencil icon and saving the changed value.