No longer supported as of 6/28/17, more information can be found here.

If you need to change your Dropbox master passphrase for any reason, you can do so via Transmit’s preferences.

• Choose Transmit > Settings in the Menubar.
• In the General tab, open the Sync Favorites popup box and choose Reset Master Passphrase

Note: If you change your passphrase on one Mac, you will need to do this on all other Macs running Transmit 4.