How do I edit an Info.plist file?
You need read/write permissions to the containing folder as well as the plist itself for Xcode to modify the file in place.

A workaround for this if you are unable to change the permissions of the containing folder is to copy the file out, edit it, then copy it back over the top of the existing file. Alternatively, you can use a different property list editor such as the CLI ‘defaults’.