You can perform operations on a variable selected in the Variables area by the standard action commands which are available in the Debug > Variables menu or from the context menu on the variables list itself. For more details about these commands, see Performing operations on selected objects.
Debug > Variables > Set...
to set the value of a variable selected in the Variables area. A dialog prompts you to enter a form which is evaluated to yield the new value for the variable. Previously entered forms are available via a dropdown in this dialog. The Common Lisp variable
is bound to the current value of the variable in the frame.