I’m not fully sure what you mean by “brush in byashna.”

If you mean “Can you brush your teeth during Beasana/Beyasana?” then:

Answer

In Jain practice, during Beyasana/Beasana, one generally eats only two times in the day within proper limits. Brushing teeth is not usually counted as eating, but care should be taken that:

• No toothpaste or water is swallowed.
• No taste-intake is done intentionally.
• One follows the guidance of one’s family tradition, guru, or sangh practice.

Spiritual Understanding

The purpose of Beyasana is to practice restraint, discipline, and control over taste and consumption. So even daily activities like brushing should be done mindfully, without indulgence.

Takeaway

You may brush carefully, but avoid swallowing anything and follow your Jain tradition’s guidance.