Once you've created a derived variable in Crunch for instance by combining the categories of a variable, you can edit that variable by selecting the properties menu. (Note the properties menu is not always visible but will appear when your mouse is near the variable card).
Then click a button that starts with Edit (it might say Edit categories or Edit combinations depending on what kind of derived variable it is).
Once a variable has been created you cannot change the type of derivation, for example you can't change from a variable created to combine categories into a categorical variable based on multiple variables. If you want to change the type of derivation we recommend deleting the variable and creating a new derived variable.