How can I add or correct EXIF lens information when using adapted vintage lenses on a Canon DSLR?

The easiest way to get some lens EXIF information, such as the focal length of the lens, is to find a chip for your adapter ring that fakes being a Canon EF lens chip. These can be programmed to push out EXIF information, as well as perform AF confirmation, and hold AF adjustment settings. I used to use the EMF chips back when I shot Canon with adapted lenses, because you could, iirc, also program it to include the actual aperture setting you were using at the time.

See: Will an adapter with an EMF AF Confirm chip work with my vintage lens?

But you can also add or edit the lens EXIF information after the fact using the command-line exiftool or its library, or any of the GUIs that use them. Because I'm a Lightroom user, the donationware LensTagger plugin is my tool of choice, because it can save off information for a specific lens (and film) combo and then reload it, so if you use a number of different adapted lenses, you aren't having to refill in all the fields over and over. But it's a bit of the PITA, as you have to also make sure to get Lightroom to update the database after you've written to the files' EXIF.