Before repairing a circuit board one must diagnose the problem and identify the failing parts. In most cases a component-level repair is required, that is, one or more components must be replaced with new ones to make the circuit work again. Most circuits are composed of passive and active components. Passive components are the ones that do not require a power source to work, for example, resistors and capacitors. A failing passive component can be identified easily but a bad chip is much harder to diagnose.
Remove all of the remaining solder at the pads using desoldering braid and the soldering iron tip. Note that the solder on the pads connected to ground will take longer to melt.
Place the new component on the board and align its pins so that they are approximately in the middle of the pads. If it is a through-hole component, push the leads through the holes on the board. Do not cut the leads yet.