A fan that has damaged blades schould be replaced immediately, because the bearing will break very fast due to the imbalance and resulting vibrations.
When changing a fan on a working system be very carefull not to touch the blades or get any cables into the running fans. Also watch out that the system is not overheating during the changing process.
Watch out that the plugs are correctly placed on the baseboard. As a short circuit or misalignment of the pins will damage the baseboard.
Open the case according to the Opening/closing the case chapter of this wiki. First disconnect the fan from its power source and make sure the cable is not stuck anywhere.