Have you got paid RHEL support? If not, installing RHEL may not be a great idea as support/updates are paid subscription based. CentOS 7.3 would be recommended instead if you do not have a paid subscription (since then you can get updates)
To install RHEL, the first thing will be to free up disk space and that can be a risky solution, as such I'd suggest using an actual pre-written guide given there is inherent risk in this. You'd then need to set-up the installation media, be that CD, DVD, USB or something else. I am assuming you got an .iso? Which is a CD image but it usually also contains a USB image within said ISO that you could use instead if you wanted to use a USB flash drive instead. It may initialize (I.E. wipe) said USB if you were to use a USB.
Redhat supply the linux commands, I generally do this stuff from linux and so not familiar with the windows versions/commands.
https://access.redhat.com/articles/142303