Hi Tracy - welcome to
RP!
Your main problem seems to be different aspect ratios. A 6x10 doesn't have the same aspect ratio as a 4x5. Here are a couple of suggestions you might try.
1. You can select constrain proportions, change the 10 to a 5 and your image will go to 3x5. Then you can change the canvas size to 4x5 and get a print with borders that need to be trimmed off but will contain all of the image in proper proportion
2. You can select the crop tool, size the tool to 4x5 and decide which part of the image to crop out.
There's also some info in
this thread about aspect ratio.