I tried Mandrake 9.1 but went back to 9.0. Assuming you have a bootable cd, when it boots it should give you the option of upgrading or installing, or somthing along those lines, and basic or advanced. I've never tried the "upgrade", and I've been advised to always do a clean install. I always back up necessary data, use advanced mode (for re-partitioning), and when it asks which partitions to format (it automaticaly selects any newly created and /), I format all just to be sure its all clean. So no "uninstall" necessary, but all data will be lost, because formating "/" is always done. That's what I suggest. Also, maybe I didn't give Mandrake 9.1 enough of a chance, but I realy like Mandrake 9.0.
Hope that helps.