You cannot install another version of IIS on your machine. IIS is tied to the OS version and has been for a while.
Note that the version of IIS shouldn't really matter in most cases. Furthermore IIS 6 is out of support because the OSes it ran on (Server 2003, XP) are no longer in support either. Therefore you shouldn't need to support this old version.
If you really, really need IIS 6 then set up a virtual machine running one of the unsupported OSes. That would be the only workaround.