You can modify the value of any variable or expression via dialogs. You can access the modify dialogs via the editor context menu or one of the variable window context menus.
For numbers you will be presented with this:
Just enter the value to which you wish to set the Number. The current value will already be present when the dialog is displayed.
The new value may be any expression that could appear on the right hand side of an assignment statement with the variable or expression on the left-hand side. So if the variable was count, the new value could be 0, a + b, or any other numeric expression. If the variable/expression was STR(A$,2,2) then the new value could be "AB", HEX(6061), B$, STR(C$,2,2) or any other string expression. Note that a string literal must be enclosed in quotes in the normal KCML fashion.
In this example the numeric variable count which currently had a value of 10 is being changed to the value of the variable oldcount + 1.
For string you will be presented with this:
This is the Large Variable window. This allows you modify any byte of the displayed string.