Fix a copper pipe leak
If this is soldered copper slip joint fittings there really is no other choice other than cutting out the faulty parts and starting over. 

Sometimes when undertaking a repair like this it can be advantageous to incorporate a UNION fitting into the line to permit the last connections to be joined together. A union fitting looks like this.



When assembling the union into the piping system do not forget to slide the brass nut part onto the pipe before soldering the unthreaded coupling part onto the pipe end.