I run OS 10.10.5 on Macs. I have teX Live Utility version 1.21. When I run it I get a window that says :Repository has a different TeX Live version, The repository at http://mirror.ctan.org/systems/texlive/tlnet has TeX Live 2015, but you have TeX Live 2014 installed. You need to switch repositories in order to continue." I do not know what this means. What do I do? How do I "switch repositories"?
You can change your repositories with Configure > Manage Repositories.
Note that one can only update to TeX Live 2015 (which is highly recommended, as it contains many improvements) by downloading a fresh installation of MacTeX.