"/usr", "/usr/local/", "/sbin"

depends how much time and effort you wish to invest. A clean install is always best if you have the time and patience to rebuild everything. The quick solution is to modify your PATH to point to the versions you want. Another option is to create symlinks to point to the desired versions. Again the best method is a completely clean install

The bad news is that there isn't a quick fix.

The good news is that MySQL isn't usually too messy as to where it installs itself. I'd advise you to clean them up by hand. Start by compiling a list of each installation. The following command should catch everything:

INSTALL_PATH="/"
find ${INSTALL_PATH}/{bin,sbin,share} -name 'mysql*' -type f
find ${INSTALL_PATH}/{lib,include,share} -name 'mysql' -type d

Repeat the process with INTSTALL_PATH as /usr and /usr/local as required. Then set about deciding which install you wish to keep and wish are going to be deleted.

Obviously one of the install will be from RPM, so you shouldn't delete that by hand. Use the package manager. And don't of course delete any data_dir's or logs without checking their contents first.