Here's a complete guide on how to repair a leaking joint whether it wasn't soldered correctly or it just started leaking in time, this guide will assist you in repairing it the CORRECT way, not the quick-fix way :)
DISCLAIMER: Got2Learn is NOT responsible for any damage done to a property of which the plumbing wasn't done by a professional, I do not recommend doing your own plumbing if you are unsure about what you are doing, always hire a LICENSED contractor when doing any type of plumbing so you can be covered by insurances if something does happen, these videos are for entertainment purposes only!