All you have to do is replace the Tremulous executable. Linux users have two good clients to choose from: an older and stable Tremfusion client or GrangerHub's client that's under active development, but is in alpha state.

If your architecture is supported and you have ncurses 5 installed, you can try these:

• Tremfusion-0.99r3-linux32.tar.bz2 (i686)
• Tremfusion-0.99r3-linux64.tar.bz2 (amd64)

Clone Tremfusion, check out tag 0.99r3 and compile. You'll likely need to install some dependencies. There is no list, but it should be obvious what's missing from compilation errors.

See their installation guide.