Perhaps you refer to a Screen Protector...? These are usually either a very thin tempered glass, or a thin plastic sheet.
To remove a tempered glass screen protector, simply place a fingernail beneath a corner of the glass, then very gently - and very slowly - lift the glass sheet from the iPad screen. The adhesive will slowly relax - allowing removal.
A plastic screen protector is removed in much the similar fashion. Simply peel, slowly, from one corner.
Once removed, the screen protector should be discarded.