Washing a woollen jumper needs care, but a few simple rules will help to keep them in good shape.
It is possible to wash jumpers by machine, but for a good hand knit jumper I choose to do it by hand, it doesn't really too long, and I think of all the time it takes to knit a jumper it is worth a little extra care.

Use cool water, not hot, and avoid sudden changes in temperature.
Use a gentle wool washing liquid something like woolite/euclan/ even a baby shampoo will work
Dont wring or stretch the jumper too much, be gentle whilst washing.
Rinse with lots of clean water
Dry flat on towels away from strong heat, but with lot of airflow changing the towel and rearranging the jumper, shaping it if necessary.

Good luck