Depends - unless the glue is old, flaky, dried, or contaminated with dirt, you don't necessarily need to remove it. Pick off big globs and go to town with regluing.
If the glue is bad on the rim, google "remove tubular glue" or something like it and pick your favorite. It'll involve some kind of solvent, and lots of time/elbow grease.
If the glue on the tire is bad, there's way less info about it. I had to search for just that recently, and came up with very few resources. The one article I found suggested that if you think you need to re-use tires, you have to weigh your options: either the glue is good, and you and glue over top of it and it's no problem. If the glue is bad, you have to assess how bad. Either it's workable for a re-glue job, or you need a plan B. Apparently removal of glue is bad for the tire. So, you can either toss the tire, or send it off to tire-alert (Google it) and have them remove the old basetape and install anew.
In short, if you need to re-use a tire, it's probably good to just reglue it as is. Otherwise, it's likely to wear out before the glue on the tire itself goes bad.
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