You've already validated shifts but want to modify them? Don't worry, it's possible!
You have two ways to modify already validated shifts, go to Schedule!
First option: recommended if you have only one or few shifts to modify.
Click on the shift
Click on "Devalidate this shift" and make the new changes
Second option: recommended if you have multiple shifts to modify.
Click on the validation button at the top right of the schedule
Uncheck the shifts you want to modify
Then edit your shifts as you wish