Stats are "maxed out" by using all your EVs in the stats. If you have already used the pokemon in battle you will have picked up EVs, probably in the wrong stats. You can reset the EVs with certain berries as listed here: http://pokemondb.net/ev
After that you need to only battle certain pokemon that give the EVs in the stats you want, see the link above for more information and links to which pokemon have which EVs.
Also, it's very unlikely you will get the maximum listed in our pokedexes - they are based on "perfect" IVs which almost never happens.