With the volume button. If that's not enough, get a more sensitive earpiece - it'll give you more volume with the same signal level. Careful, though - decent hearing aids run about $3,000 each and, even if the damage is only to one ear, you should wear 2 to be balanced. (And medical insurance usually doesn't pay for decent ones.) Oh, and the tinnitus that hearing loss causes can drive you batty at times.