BCH to BTC in same Trezor wallet
If you sent BCH to your Trezor BTC address or vice versa, you can use https://trezor.io/recover-coins/ to recover your coins.  The only exception is if you send BCH to a BTC 3... address (segwit), since these are not supported by Bitcoin Cash.

To convert BCH into BTC or vice versa you need to use an exchange.