Feb 24, 2018, 14:4502/24/18
Hello!
You must have sacrificed Champions to upgrade the Rank of other Champions. This is another type of activity.
To get that quest completed you need to sacrifice Champions for XP. This means that you should sacrifice unneeded Champions for another one who has not reached maximum level yet.
The Upgrade tab should look this way with a green "Sacrifice" button:
I hope this was helpful!