Migrate contacts from other systems
First of all download your contacts from the old system, in this example from Owncloud. The format should be vCard or CSV (comma separated)
Save the contacts file on your harddisk
Now you need to open your Webmail and go to the Contacts section. There click on Import on the top of the screen.
In the upcoming screen select the file previously downloaded, select the addressbook where to import your contacts, select the group setting and define if your existing contacts should be replaced
Now click Import
Depending on the amount of contacts the import need some time
At the end the import log schows you which contacts were imported successfully and which one was skipped due to a problem. For the skipped one you need to interact manually.