Currently it's recommended to download an image for the new release and install it rather than upgrading your device. However, upgrading is still possible and <i>should</i> work normally but we are not responsible for any broken installations.
As an example, to upgrade from the 20.05 to 21.03 release of postmarketOS:
# sed -i -e "s/v21.03/v20.05/g" /etc/apk/repositories
# sed -i -e "s/v3.12/v3.13/g" /etc/apk/repositories