There are 2 things you should have in mind about TFS 2015 Upgrading.
1 The database upgrading process will cost much more time than upgrading among Update package of TFS 2013.
2 The dedicated SharePoint integration package is removed and M$ did a shit test again. If you, like me, installed TFS on a server other than SharePoint cluster, now you have to install the whole TFS on all SharePoint servers. After the installation, the upgrading wizard will be popped up but it will failed due to no database can be found. You have to choose to install SharePoint integration role yourself. And after that, you will know that TFS SharePoint integration package 2013 will NOT and NOT ABLE to be removed, lol, just another piece of crap.