Move data from 12.04 to new 15 server

Our company has been using Nextcloud now for about a year and we’ve been happy with it. From the IT side, no one is a Linux person here but it was easy enough to follow instructions to get the vm up, connected to LDAP and running on 12.04. Unfortunately, our vm has run out of storage space and we can’t even do an upgrade. I purchased the larger capacity vm this time around (500Gb) and am curious if there is documentation that details moving all our data over to a new v15 vm from v12.04?

you find a description of backup&restore here:
after that on the new maschine you can upgrade from 12.04 to 15.04.