Five options:
a) Copy the file old release file (.img.gz, .img, or .tar) to the Update SMB share on the box and reboot
b) Use an SFTP client to copy it to /storage/.update, then reboot
c) SSH in, change directory (cd) to /storage/.update and then wget the file, then reboot
d) Use the manual update function to change to an older release series and select an older version, then reboot.
e) Create new install media using the older .img.gz file and clean-install using that.
Unless you did 'E' once you're on the old release, do a hard reset in LE settings to remove all traces of newer versions.
All of the above are documented in the wiki .. and I'm done with the spoon feeding.