As of Generation IX, the only way to modify IVs on an existing Pokemon is to Hyper Train them. Your Pokemon must be at Level 100 (prior to Gen IX) or at least Level 50 (Gen IX) to be Hyper Trained. Once prerequisites are met, giving the game's Hyper Trainer a Bottle Cap will max one IV and a Gold Bottle Cap will max all six.
Do note that Hyper Training doesn't actually change the IVs -- it merely modifies how the game treats them (akin to how Nature Mints work). This is important in the context of Hidden Power and passing IVs via breeding.
