If you enabled sheet protection, you can remove it at any time. (You will recognize active sheet protection by the fact that the icon has a dark color.)
To do so, choose the ribbon command Review | group Protection | Sheet protection 
again.
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. However, PlanMaker still remembers them. Thus, if you re-enable sheet protection, the same protection settings will become active again.
Tip: If you save a document with sheet protection enabled, sheet protection will be automatically re-enabled the next time the document is opened. If sheet protection is still secured with a password, it can only be disabled with the correct password. This allows you to protect documents against changes by unauthorized persons.