It depends on which "old look" you are talking about. If you got stuck in mobile view and thought it was a "new look", then just go to an article page and scroll all the way to the bottom. There should be a button to "view full site". If you are already using desktop view and are referring to FandomDesktop (which has a collapsible right rail) versus Oasis (which does not have a collapsible right rail) versus Hydra (the default skin for Gamepedia wikis), the you can switch back using your profile preferences. Go to Special:Preferences and open the Appearances tab. Under the Skins section, select either the Oasis or Hydra option (depending on whether or not this is for Gamepedia wikis) and save the change.
Note that there are about 300 wikis where the default skin has been set to FandomDesktop. The skin preference for those 300 wikis, Gamepedia wikis, and the rest of Fandom are managed separately. So if you want to avoid FandomDesktop on all wikis, you will need to change your preferences using at least one wiki from each of these categories. Also note that all older skins are slated to be removed in a month or two. At that time, you will no longer have a choice and have to use FandomDesktop.