Cmake does not provide an automatic uninstall target to go with the install target it provides, for some strange reason.
(Whereas autotools and many manually-written Makefiles, do.)
It is not such a big deal since the common way of handling software is via package managers which do that for you.
But it should not be a big deal to install a new version on top of the old version, since it will tend to overwrite everything. (And anything it doesn't overwrite, will most likely be ignored.)