I recently updated from Ubuntu 16.4 to 18.4 and after the reboot at the and of the update, the screen blacked out, every time i restart the cpu its the same, just blacks out and i cant even move the mouse. only way o can do anything is access grub and root mode
Essentially, you cannot downgrade an existing Ubuntu system. The only way to downgrade is to perform a fresh install, i.e., an install where you wipe out the existing install first.
A fresh install wipes out all system files, but also the user data if these are located on the same partition. If needed, you may want to save existing user data on your system first from a live session, before starting the install.