How do I brush the back teeth without getting bitten?

Reaching straight into the mouth from the front puts your fingers directly in the path a dog would naturally close its jaw. Approaching from the side, lifting the cheek and lip with one hand while the brush enters from that same side, keeps your fingers well away from between the teeth even if your dog does close its mouth unexpectedly.

A dog that is comfortable with front teeth brushing is far less likely to react badly when you extend to the back molars, so working through the front teeth confidently for a couple of weeks before pushing further back gives your dog a chance to stay relaxed the whole way through.

If your dog tenses, pulls its head away repeatedly, or starts to stiffen as you move toward the back teeth, ease off and finish there for the day rather than pushing to complete every tooth. Ending on a calm note, even with partial coverage, keeps tomorrow's session easier than today's.

Smearing a small amount of toothpaste on the back molars directly with a finger, then following up with the brush a few seconds later once your dog has relaxed into licking, sometimes creates a smoother transition into brushing that specific area than going in with the brush cold, especially for a dog that tenses the moment it sees the brush approach.

If your dog consistently turns its head away the moment you approach the back molars specifically, rather than resisting the whole session, this localized reaction is worth mentioning at a dental checkup in case there is discomfort concentrated in that particular area.