You set the value of the global variable with the Set function. Set( MyVar, 1 ) sets the global variable MyVar to a value of 1.
Use the Back and Navigate function to change which screen is displayed.
Reference:
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/working-with-variables
https://docs.microsoft.com/en-us/powerapps/maker/canvas-apps/functions/function-navigate