Hi Bosco,
Thanks for posting in our community.
As your description, if my understand is right, you want to add multiple versions to a old document.
Firstly, you should confirm you have open the Versioning function.
You can use Versioning to view or restore an old version of a list or library.
You can find it from site-> Document->Setting-> library Settings->Versioning settings
After setting complete, you just need to upload your new version of document.
SharePoint will prompt as shown below.
Click Add file and the latest version will be added to the old document. You can see all versions from Version history.
Please feel free to post back to us if the problem persists and we will try our best to help you.
Regards,
Jason