There is a way to do it and it’s a little fiddly. It involves removing a folder in your Steam directory and then utilising Steam Achievement Manager. This video below shows how to do it but you will have to reset up all the Resident Evil 2 settings. It basically wipes any info from a previous run-through of the demo.
Using the Steam Achievement Manager could be risky so use this with caution as you don’t want to find your main Steam account banned by Valve. Might be safer to set up a dummy account first.