What kind of variables? What's wrong with the player using the Restart menu item in the UI bar?
If you're simply referring to story variables and you've removed the UI bar somehow, you can show the player the standard restart dialog via the UI.restart() method. For example:
<<link "Restart">><<run UI.restart()>><</link>>
Alternatively. If you'd rather not use the standard restart dialog, then you may use the Engine.restart() method. For example:
Restart now? Unsaved progress will be lost.
<<link "Restart">><<run Engine.restart()>><</link>>
Though, as shown, I'd be explicit with the player about what's about to happen because there will be no prompt.