How do I change the root location for Bash in Windows 10?

Not yet!

In Win10 Fall Creators Update, we've added the ability to install one or more Linux distros (Ubuntu, openSUSE, SLES, Fedora & others coming soon). Alas, we were not yet able to wrap-up the work required to enable distros to be installed/moved to a non-system drive. We'll be looking into completing this work in a future release.

Now it possible with https://github.com/DDoSolitary/LxRunOffline

choco install lxrunoffline

• No Internet access required when installing.
• Install any Linux distros to any location on your disk.
• Moving existing installations to other locations.
• Duplicating existing installations. (To create backups.)
• Register existing installation directories. (For portable usage.)