First, go into their github and download it (if you don't know how to download from github, just join their official discord server, get to "1.5 testing" section, go into "testing-announcements" channel, and there's a link for download of latest build).

After you're done downloading the build itself, you need to unzip "CombatExtended" folder into RimWorld "Mods" folder. To find it you need to go through those steps: (your steam folder location) --> steamapps --> common --> RimWorld --> Mods

I hope this helped