The ability to install more than one major version of Nik was introduced in Nik6 if I remember correctly. This means that you can install Nik7 next to e.g. Nik6.
Nik 5 has no such possibility. If you want Nik5 and a newer version, you’d need to install Nik5 first and add the newer version afterwards.
CAVEAT: Please check if Nik7 can co-exist with Nik5 before doing anything…and make sure to have the Nik5 and 7 installers within reach and possibly also the Nik Cleanup tool.