I just changed my battery and my iphone 6s won’t close right and tight in the upper right hand corner. My iphone seemed to fit tight before, and it’s not the “yanzy” screen because its the same screen and I do not think my phone was ever bent. There are remnants of two colors of glue/tape strip, both black and white and I would have to best guess this suggestion as the most likely to be the problem. I have tried refitting it a few times making sure to slide that top edge under, but no success. I bought this phone used, so this may be the third battery easily. Instructional videos for the battery replacement rarely cover problems with the glue or how to replace glue strip.