Rear brake caliper piston will not re-track.

Here is a video that will help with this:

https://youtu.be/hPzxdnLJbhc

and

https://youtu.be/j8YwfSKEzZo

If it will not retract then you are going to have to replace it as it is seized or the fluid is not being allowed to return.

Here is a guide that will help replace the brakes and caliper as you will most likely need to replace both while you are there:

https://www.2carpros.com/articles/how-to-replace-rear-brake-pads-and-rotors

and

https://www.2carpros.com/articles/brake-caliper-replacement

Below is the process for this as well from the manual. Let us know if you have other questions. Thanks