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Downgrade AUR package
First, go to the directory containing the PKGBUILD file for the package. If you're using YAY (as you should), that path is ~/.cache/yay/<package-name>.
Once there, as there is a Git repository present, just reset the tree to your desired version.
E.g., to go to the previous version:
git reset --hard HEAD {#1}
Then, build with Makepkg:
makepkg -si