Winter, that is July/August is the time to re-pot all bonsai.
You simply MUST do it at least every 2-4 years or the plant may get sick and might even die.
What do you do? Gently tip the bonsai pot on its side and cajole the plant out of the pot by wriggling its trunk at the base. Maybe even push up through the drainage holes of the pot with some wooden doweling.
Remove about 1/3rd of root ball by gently raking it off with a sharpened chopstick or a stout knitting needle.
Re-pot the bonsai into its original pot or into a slightly bigger bonsai pot. Use good quality potting mix or bonsai mix and water it in well. Now place the plant in a semi-shaded area for 3 weeks to let it recover.
Put it in full sun after about 3-4 weeks.
Job done!
Hooroo!