Use the
ConfCenter or edit the yapb.cfg (cstrike\addons\yapb\config\):
// Determines minimum bots skill, when creating bot with random parameters.
yb_minskill "80"
// Determines maximum bots skill, when creating bot with random parameters.
yb_maxskill "100"
Change these settings to 40-60 for example (try the best settings for you).