Most likely, you can't.
I am going to assume this happened recently. If you recently sent BCH to a BTC address, then most likely it was sent to a segwit enabled address. If that is the case, they are probably gone and locked up forever.
IF you sent them to a legacy address, they will show up in the wallet. BUT, there may be more problems. If the client that contains this wallet is segwit enabled, you CAN'T and should not attempt to just send them back without first disabling segwit.
The best scenario, is they were sent to a legacy wallet address on a legacy client. i,e. no salt, no segwit, etc. Then you may recover them.
No matter what the case, it is worth at least trying. You can try with one of these. https://bch.btc.com/docs/help/bch_segwit_recovery