Do you want to replace the old LEDs or simply re-solder them? How are your LEDs attached to your heatsink, with thermal glue or screws? You can order new LEDs from a number of venders, I've had good luck with Rapid LED. If you are going to replace the old LEDs first use a soldering iron to remove the electrical connections. Then simply unscrew them from the heatsink if they are screwed on, chisel them off if they are glued on. Then first tin the LED connections if they are not already tined (add a small amount of solder to the connection using floss). Then solder the LED pucks to the electrical connections, being sure to connect to the correct positive and negative connections. If the LEDs were screwed on apply a thin layer of thermal grease and then screw them in place. If they were glued on re-glue them with two part thermal glue. There are some good how to You Tube videos available.