How do you update the YUM package manager repositories?

You did not specify the distribution you are using. I guess it is rhel/centos 5 or 6: if so, you just need to add the proper EPEL repository to your YUM configuration and then

yum update
yum install tmux

No need to download/compile it manually.

It is a bad idea to install "unofficial" software, specially on "enterprise" distributions or any sort of machine with critical jobs. You are taking over the full responsibility of security (and other) fixes, and keeping it in synch with the rest of the system's software (libraries, configuration, security configuration, ...). Look for alternatives, check what your distribution might have to say on the particular package. They might be excluding it due to unsolvable problems of some sort, perhaps licensing (that might or not be a problem to you, as a final user or due to the legal situation in your jurisdiction) or even security problems.