How to switch from libinput to synaptics in Ubuntu 18.04

I had the same problem and basically solved it by following Ryko's advice.

sudo apt install xserver-xorg-input-synaptics

... did the trick. I didn't uninstall libinput like Pilot6 said and libinput is still happily handling my keyboard, etc.

If you're running an HWE release (like 18.04.2) then you'll actually want:

sudo apt install xserver-xorg-input-synaptics-hwe-18.04

You don't need to remove any packages, just add this and once it's active you can make adjustments to your taste. (as others have indicated above)

Hint: The suffix "-hwe-18.04" needs to be appended to any pkg that HWE impacts. Read up on HWE and you'll understand why and the scope of the packages impacted.