transfer scores to a new computer

Install MuseScore on the new computer.
Find where the scores are saved - run MuseScore and Edit >Preferences >General contains the path to the Scores folder.
Copy from the old machine to the new machine.

Just copy the files to a thumb drive.
Or use dropbox, box.com, GoogleDrive, OneDrive or some other cloud service
Or upload all to musescore.com

Same as the way to copy Word files or any other files, really. If your OS doesn't handle that for you - Windows at least does - a flash drive is usually the easy way. On Windows, if you don't let it do the job automatically via OneDrive, you can just copy the contents of your Documents folder.