@1stWorldProblems My computer at my Prometric center just had a Windows operating system, and my test screen was already on the LSAC login. All I had to do was type in my username and password. Hopefully that assuages some of your concerns!
Most law schools have a way to for you to send updated materials through a portal or via email. You can just contact them saying you've had a change on your resume and you'd like to send in the new document. This is very normal. I'd do it asap though.
If you were using the old PT's at all I also have a document that shows which old test sections they used to make the 101-158 series. If that would be of use to you send me a DM and I can pass it along to you