I ran across this article by the U.S. General Services Administration. It includes a nice, step by step procedure on how to fix holes in a metal roof.
This procedure includes guidance on patching holes in a sheet metal roof by brazing and welting. Generally, this work should be accomplished by a experienced roofing contractor.
The article warns not use asphalt roofing compound or cheap aluminum-based roof coatings to make the patches. These repairs seldom last, and are hard to undo, and are potentially damaging to the existing roof.
For the full article, see: https://www.gsa.gov/portal/content/113138