If you enabled sheet protection, you can remove it at any time.
To do so, choose the ribbon command Review | group Protection | Sheet protection 
again (you can recognize an active sheet protection by the fact that the icon appears darker).
The sheet protection is then disabled again.
The protection settings defined in the dialog box of the ribbon tab Home | group Number
are now ineffective again. But PlanMaker still remembers them. So when you re-enable the sheet protection, the same protection settings will become active again.
Tip: The current state of sheet protection is saved within the document. If a password has been assigned, users will be required to enter the password to disable sheet protection. This way you can protect documents against changes by unauthorized persons.