Is the pipe coming out of that T going to an outside hose bib? If so, it looks like it is threaded on. You could remove the hose bib which would loosen up that arm. If there's any leaway to the right, you could then unsolder that joint on the left, the either clean it up and resolder, or pull that T out of there and replace. If you can't slide the pipe to the right, then you'll need to cut it. you can put it back together with a repair coupling. this looks like a fitting that you'd use to connect two pieces of pipe together, but omits the stop in the center. You slide it on one side, then bring the two ends together, then slide the fitting so it is evenly over those two ends and solder it. If you can free up that arm, then you'd need another repair coupling and cut it off, then repair.