There are multiple options:

• The very extreme: Uninstalling Google+ as described by Miguel's answer. This will work because the new "Photos" app is part of Google+; an uninstalled app can no longer be the default one.
• Installing another gallery app (there are plenty of nice alternatives available, like e.g. QuickPic). As this introduces an additional option, the system will ask you again which app to use.
• Go to Settings→Apps, switch to the "All" tab. Scroll to the Photos app (if not found, try the Google+ App), open its entry, and "clear defaults". No additional install/uninstall required, this will simply "undo" your action as requested.
• Take a look at Default App Manager, which allows you the same in a more convenient way (e.g. much easier when you don't know which app holds the default).

There are multiple options:

• The very extreme: Uninstalling Google+ as described by Miguel's answer. This will work because the new "Photos" app is part of Google+; an uninstalled app can no longer be the default one.
• Installing another gallery app (there are plenty of nice alternatives available, like e.g. QuickPic). As this introduces an additional option, the system will ask you again which app to use.
• Go to Settings→Apps, switch to the "All" tab. Scroll to the Photos app (if not found, try the Google+ App), open its entry, and "clear defaults". No additional install/uninstall required, this will simply "undo" your action as requested.
• Take a look at Default App Manager, which allows you the same in a more convenient way (e.g. much easier when you don't know which app holds the default).