Once you have created a Show or Scene Variable (see: Using Variables), you can use the Update Show/Scene variables function.
It allows you to change the the value of a chosen Variable as an action.
A number Variable can be updated to another number. A text Variable can be updated with text etc. This is useful when you are creating sequences of events that need to happen in a certain order, OR if you want to create a Dashboard widget that changes Variables (see: Creating a Set Variable in the Dashboard)