Open /Bully/Config/dat/pedstats.dat in any text editor (notepad++ being my personal preference) and simply edit the stats. At the top of the file, Rockstar left information regarding what each of the stats does in the game. This file sets the default values for ped stats, but the stats can be changed in scripts and/or missions.
If you want to change the stats of individual peds in a LUA script, this can be done as well using GameSetPedStat, or PedOverrideStat. They both have the same effect, and I haven't noticed any differences between the two. They both take 3 arguments, the ped to set the stat on, the stat ID, and the new value for the stat. For example, GameSetPedStat(gPlayer,20,200) or PedOverrideStat(gPlayer,20,200).
The stats are the same for the scripting functions as they are in pedstats.dat, however you need to use a number ID to reference a certain stat. It's pretty simple to get the stat number from pedstats.dat. A will never be changed, so it has no number, as it's not technically a stat anyway. B is 0, C is 1, D is 2, E is 3, F is 4, and it keeps going like that down the file. Just start with replacing B with 0, and label the rest of the lines in the file counting up from that 0.
There is also a function GameGetPedStat, which takes the ped and stat id, and returns the current value of a stat for that ped. For example playerAnimSpeed = GameGetPedStat(gPlayer,20).