[ubuntu] keystroke combination to downgrade the Ubuntu version

Hallo,

I use ubuntu with vmware. After the last ubuntu-update, ubuntu does not start propperly.

As I had these problem some month ago, I know that there is an easy way to downgrade to an older version.
The problem is, that I am not able to google the keystroke combinations which I need for this.

Hope you can help me out and tell me the keystroke combination to use the old ubuntu version.

Thanks from Hamburg (Germany)

Sebastian