Ken, If I were you I would remove the metal stays (if your pack has them) and shake all the debris out of the pack (or turn the pack inside out and brush it all off)... first I would attempt to clean it just using a scrubbing sponge and water and maybe a very gentle detergent - a really small amount mixed in with the water, not placed directly on the pack. Make sure to rinse with clean water after. If that approach failed I'd gently agitate it in a tub of warm water/small amount of mild detergent and then repeat with clean water until there is no suds/dirt. After that, wipe down the pack as thoroughly as possible with towels/paper towels and then lay outside to dry. Do this on a very hot sunny day!
Should work ok. Of course, my pack has straps and a hip belt that are not excessively padded so it dried great.