Main menu -> Tools & Extras -> Steam Workshop (this will open the mod manager). From there you can load/unload mods. Note that some mods (mostly the more complex ones that aren't dropdown mods) don't load/unload correctly because the mod manager is slightly bugged I guess.

Dropdown mods (ones that appear yellow in the mod manager) can be loaded in the lobby: there's a dropdown list you can select on the right side near the other settings.