Instructions for replacing the seal are here: iPhone Display Adhesive Replacement
I just had the same experience. The blue backing would not separate from the adhesive.
I tried a new tactic: Remove the ping backing and apply to the case, and then remove the blue backing. No go. It stuck to the blue backing more than it did to the case.
I should have just left the remains of the old strips in place. Instead, I just installed the battery without the adhesive. I hope that’s good enough.
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