Hello Kaar,
Besides downloading the latest DB2 upgrade guide, have a look on this note which is a good starting point. Well, this note is also stated in the guide:
1837312 - DB6: Upgrade auf Version 10.5 von DB2 für LUW
From that note there are references to IBM Website and useful DB2 SQL queries which should be executed.
Good luck with your upgrade project!
Serhat