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:
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.