Several factors will decide whether you should go for repair or a complete change of your roof. If it is a matter of a few leaky patches or damaged areas, a repair of those areas is an obvious solution. If a few of the shingles have blown away as a result of a storm or strong winds, you should definitely go for replacing only that part of the roof. Repairing and replacing a few shingles will surely be a cost efficient solution as well.
However, if your entire roof shows signs of damage, it would be a better decision to go for complete roof change to save on recurring repairs.