In order to change the version number of your theme you first need to extract all the files and edit the manifest.json file in a text editor like Notepad. You can change the version number to basically anything you want, it just has to be different from 1.0.
Once you are done changing the number, save the file and then put all the files you extracted into a Zip file or XIP file if your computer can do that. Then you can upload it.
If your computer doesn’t have the ability to unzip XIP files, you can either download a program that can extract the files or maybe just export it again as a ZIP file and go from there. I ended up downloading a file with the ability to open XIP files but now I just export them in ZIP format instead.