Upgrade at Amazon
Most of the time you can just go to More Purchase Options and purchase the upgrade. Amazon is working on combining their movies so they don't have a different listing for UHD and HD, but there are still a lot that are not combined. Use the Amazon 4K Spreadsheet if you cannot find the link for UHD movie to purchase.
If for some reason Amazon shows you own the movie and you still want to repurchase it, you can follow the directions below:
1) Delete from Library
https://www.amazon.com/gp/video/mystuff/library/ref=mycd_redrct
Find the Video you want deleted and hit Delete underneath the video
2) Go to Manage, Deleted items
Permanently delete the video
3) You will then be allowed to repurchase the video
To see your entire library in a little easier format go here: https://www.amazon.com/hz/mycd/digital-console/contentlist/video/dateDsc/