You can unlink a photo just by using your operating system to move the photo.
Instead of spending time linking individual images, go to the folder tree and right-click on the topmost folder and choose "update folder location". Then you can show Lightroom where it should be looking for the folders. When you update the top folder all of the subfolders will be updated as well.
I clicked on the little rectangle on the upper right hand corner of the
first image in the LR catalog. It asks
Do you want to locate file? I click "Locate" and point to the original
file. Typically, ALL the images in the LR catalog are also automatically
located, all the little rectangles disappear. In the current situation,
only one image is located, not all the others even though I checked the box
"Locate nearby missing images". So, am stuck now!
Sounds like you didn't read Jim Hess's message at all, he had some good advice.
Are all of your images in a single folder?
I fixed it thanks to all of you!! I removed the photo that was wrongly linked from the original file to unlink it. Then I linked the LR image to the correct original. All the other images automatically linked!!