Devprev, or "developer preview" is a Foldit Update Group. Devprev is used to test software changes before they are released to the "main" update group. Using devprev is voluntary, but it can be very helpful to the Foldit developers.
Changing to the devprev update group requires editing the file "options.txt", which is found in the directory (folder) where Foldit is installed.
Foldit should not be running when you change the update group.
Find "options.txt", then open it in Notepad or the editor of your choice.
Locate the line:
"update_group" : "main"
and change it to:
"update_group" : "devprev"
Save the changes to "options.txt", then start the Foldit.
After you switch to devprev, when Foldit starts, it downloads and install new updates through the normal update process.
The Foldit General Options panel shows the update group and the specific software build (build ID) that Foldit is using.
Many players have Foldit installed multiple times, and use a different installation to test devprev changes. See Multiple Foldit Clients and Foldit Multi Client for more on running multiple copies of Foldit.