Place the new LED through the open holes

The LED should cover the schematic symbol on the circuit board and the flat edge of the LED should line up with the tip of the diode symbol shown by the arrow

If the LED does not stay, you can tape it in place on the opposite side of the board

Warm up the trace on the circuit board and the LED lead with the soldering iron

Wipe your soldering iron on the damp sponge every time you pick it up

Apply solder to the lead until the hole completely fills with solder