With the advent of the skirmish feature (terrific) maxing out 10 guardians at a time, the guardian upgrade process needs revised.  It's the most cumbersome process in the game:

scroll down

click on guardian to be updated

click on upgrade

click on guardian to update

click on increase rank

click on yes I'm sure

Click on ok

Finally done

Click on info

repeat from above to do the 2nd guardian

Thanks