Can only go forward ... not backwards ... so no, no way to downgrade a 3.9dev to 3.8.x highest.
Have you developed courses you'd like to keep? IF so, might use command line backup.php script in code/admin/cli/ and save the backup .mbz file out where you can see it by humanly recognizable name - like /home/clibackups/. Script does not require you to login.
Wipe out the 3.9dev. Install a fresh 3.8.highest. Then attempt to restore the backup. Normally restores of courses from higher version to lower version have issues, if they work at all, but in this case your versions are close ... so you might get by.
'SoS', Ken