How can I update Adobe Acrobat Reader on my Mac?

FWIW, if you already have Reader installed. Select "Check for Updates" from Reader's Help menu. Follow the instructions.

Here are your Acrobat Reader DC options:

1. Follow dialabrain's advice if you installed Reader after macOS Catalina.
2. If you installed Acrobat Reader prior to any update to Catalina (10.15.7) or later, you probably want a current version, as I am uncertain if a really old version will do this update for you. Just click the Download Acrobat Reader button.
3. If your installation of macOS is older than macOS Catalina, then the default version from the Download button is not the correct one, and you need to click the tiny print link above that download button and select the older operating system from the panel that appears.

See: https://helpx.adobe.com/acrobat/kb/install-reader-dc-mac-os.html