Just updating isn't a very safe way to upgrade, use what they suggest in the mail instead:

To update the opkg package itself without upgrading the entire firmware, the following commands may be used once all repositories have been updated:

   cd /tmp
   opkg update
   opkg download opkg
   zcat ./opkg-lists/openwrt_base | grep -A10 "Package: opkg" | grep SHA256sum
   sha256sum ./opkg_2020-01-25-c09fe209-1_*.ipk

Compare both checksums and, if matching, proceed with installing the package:

   opkg install ./opkg_2020-01-25-c09fe209-1_*.ipk