Make sure you have spotted the minus sign printed on the side of the capacitor to avoid any error in the placement of the spare capacitor. If the pins of the new capacitor are not soldered in the right pin holes it will result in severe damages.

Use a soldering iron to remove the dysfunctional capacitor. Use desoldering braid and clean the PCB if you see electrolytic leakage.

With the soldering iron solder the spare capacitor to the main board with the negative terminal in the same hole as the previous capacitor.

With a cutting plier cut the terminals of the capacitor as close as possible to the main board. This will avoid any risk of circuit shortening and new damage to the subwoofer.