Yes. On a upvas (upavās) day you can brush your teeth. Brushing itself does not break the vow of fasting as long as you do not swallow toothpaste or water unnecessarily and you spit out what remains. Some strict practitioners prefer to minimize rinsing or to use only a small amount of water, but most consider brushing clean mouth without consuming food or drink to be acceptable.

Differences to note:

• Digambar and Shwetambar traditions may have slightly different practical practices for upvas, but both generally allow brushing as part of maintaining cleanliness.
• If you’re following a very strict vrata, you may choose to use minimal toothpaste or simply rinse with a small amount of water and spit, staying mindful not to ingest anything.

If you’d like, I can tailor the guidance to the exact type of upvas you’re observing (e.g., one-day partial fast vs. more stringent vrata) and point to specific Jain sources.