I recently published my novel "New Hunters at Imperial Guest House". I've finally got the Kindle version right after I downloaded Kindle Create and reformatted everything with it. However my paperback is a mess. I didn't know how to format and didn't know about the templates available in KDP so I formatted using whatever common sense I had. Unfortunately I ended up with the 8*10 format when my novel which is quite short should have been in the 5*8" format. I tried to change the format in the paperback version but I'm not being allowed to. What should I do? How can I change the format now? Please help.
Amazon will not remove a published paperback book. You'll have to remove it from sale (I don't use KDP Print Beta, so I can't tell you exactly how to do that) and republish as a new project (with a new ISBN) at the trim size you want.
The reason for that is that the ISBN is tied to the title, the author, and the format. You definitely don't want an 8x10 novel. I like 5.5x8.5 inches, which is large enough to work with but not oversized.
(Don't trust KDP to publish a print edition. Don't trust CreateSpace to publish an ebook. Each does one thing well and the other thing poorly.)