Why is the display bending?

The battery has expanded. You need to replace the battery to fix that issue.

Here is a YouTube Video on how to open the screen without damaging it, it's pretty much how I do it except I put my phone on a heat mat to heat the whole thing up to 70°C

https://www.youtube.com/watch?v=wTitXJGw...

I'm not sure where you can get this battery from other than eBay so I'll link this here if you need it:

https://www.ebay.com.au/itm/OEM-Li-ion-B...

There is a risk of replacing the battery as the screen that is held down by adhesive needs to be removed first in order to have access to the internals of the phone.

If you are not comfortable with replacing your battery it's best that you take it to a mobile phone repair shop for them to replace the battery for you.

Adhesive strips under these screens are usually only on the edges of the screen about 0.5mm in max and not under the display area.