Update 1 --- the beta wallet server is back-up - but Trezor is still working on the "coin splitting tool" for Bitcoin Cash and say it will be available after 13:20 UTC today Aug 1st....
Update 2 -- Trezor states that you should only use https://beta-wallet.trezor.io to update your firmware to 1.5.1. To use your wallet again, disconnect your device and reconnect with https://wallet.trezor.io. If you want to claim your bitcoin cash now... to send to another wallet (since Trezor is still working on theirs...) go to
https://trezor.io/claim-bch/ and fill in the BCH wallet address and amount of BCH to transfer.
Source:
https://blog.trezor.io/claim-bcash-bitcoin-cash-bch-bcc-trezor-wallet-f0a810d5864a
Currently the firmware update is only offered in beta:
https://beta-wallet.trezor.io
Update your Trezor by holding both buttons as you plug in the USB cable.
Several other nice features are in 1.5.1 - see the onscreen notes.
Follow the onscreen directions...
Have access to your Bitcoin Cash!
Source: https://beta-wallet.trezor.io