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Hello, I would like to replace my I phone 6s battery. Should I reuse the screen adhesive or do I need to replace it with new screen adhesive?

The pre-existing adhesive will be thoroughly damaged when you remove the screen. Technically speaking, you don’t need to replace the adhesive, the phone will re-assemble fine and you will not see a difference. Apple was experimenting with the adhesive in preparation for the iPhone 7 which was rated as water-resistant. However, if you replace the adhesive, you will not be able to achieve the same level of sealing as when it’s done in the factory. Full disclosure: I run a shop and I always replace the adhesive as a best practice. For DIY, it’s not necessary but preferable.