Your OS in your signature would be useful if not stated as that determines how best to uninstall.
If you’re on Windows then the best way is to use the Windows uninstall feature from within control panel. That way you can uninstall only Cubase 9.5 and not any other VSTi that you also have licences for. You may have to look for updates as well as the original installation. I’d strongly advise against just deleting all the Steinberg folders.
The reason most folk don’t uninstall an old version straight away (or before a new installation) is to give a chance to check that everything works just fine with the new version. I understand you have space concerns so can see why you might want to get rid of the old one first but I’d still try and install the new one first just in case. Some of the new installation usually replace older library versions of included VSTs so you shouldn’t assume that you need the whole disk space that it says is in the download once the installation has completed.
FWIW if it were me it would depend on just how short of space I was in the short term. If there was enough space to keep both versions I’d do so until I knew everything was working just fine (that’s what I do on my Laptop).