hi Junio,
IF you are at the prepare stage which include EXtraction,COnfiguration,Check phases , then you have to reset by using the back button functionality here . In order to activate the back button the
Gui should be at stop stage .
If your upgrade is at Preprocessing roadmap . Then you do have to choose the option
Update->Reset Update which will run some phase to clear the shadow system from abap as well as from database level .
If you are at downtime phase then you do need to restore teh system till preprocessing stage and then use the above steps to clear the system .
Thanks
amit jaiswal