How to use xdelta and download only differential update files
For those having a slow connection, it is possible to download a smaller differential file that contains only the new Alpine version changes. You can then use the downloaded xdelta file to update your previous Alpine release iso file to the new one. You will still need to download the new iso checksum file and test the integrity of the newly built iso file. For xdelta to work you should have the three files on the same directory level, i.e.: alpine-1.10.3-1.10.4.xdelta, alpine-1.10.3-x86.iso and the alpine-1.10.4-x86.iso.sha1
wget –c http://www.alpinelinux.org/cgi-bin/dl.cgi/v1.10/releases/alpine-1.10.3-1.10.4.xdelta wget http://dev.alpinelinux.org/alpine/v1.9/iso/alpine-1.10.4-x86.iso.sha1
Make sure you have xdelta3 installed and build the new iso
apk add xdelta3 xdelta3 -d alpine-1.10.3-1.10.4.xdelta
If checking the integrity of the file shows: alpine-1.10.4-x86.iso: OK then you are ready to continue.