Include:Using Internet Repositories for apk-tools
Edit the /etc/apk/repositories file using any editor of your choice (nano for instance).
Contents of /etc/apk/repositories
http://dl-3.alpinelinux.org/alpine/v3.20/main
http://dl-3.alpinelinux.org/alpine/v3.20/community
In the above file, to upgrade Alpine Linux from version 3.20 to 3.21, simply replace the number 3.20 by 3.21 in all the places:
Contents of /etc/apk/repositories
http://dl-3.alpinelinux.org/alpine/v3.21/main
http://dl-3.alpinelinux.org/alpine/v3.21/community