the EPO license from download portal works for every epo version.
So if you doing upgrade by agents, you have to do new task and deploy as force installing then it will upgrade in client side. for upgrading ePO agent in ePO, you have to do the second point which Troja mentioned.
No need of any document for ePO upgrade from 4.6. to 4.8, Only few things need to take care.
1. Make sure SQL SA password don't have any special credentials.
2. Take ePO DB and installation directory backup. follow McAfee KB for more information : McAfee KnowledgeBase - ePolicy Orchestrator 4.x installation / patch upgrade checklist for known iss...