This process took me a long time (days in preparation!), but it’s good to do it right.
What i wanted to do was upgrade my asus eee laptop from debian 7 to 8 (wheezy to jessie), so i read around the topic a lot, using google searches and of course the debian instructions, which can sometimes be quite verbose but also have a lot of good stuff. In addition there’s specific instructions for the asus eee.
For the upgrade itself i followed the excellent clear instructions at howtoforge
It took a whole afternoon, the “apt-get upgrade” alone took an hour, much longer than i had read.
At a certain point i got this horrifying screen:
So i needed to go to the debian forums to ask for help about that but ultimately it worked out fine and everything seems to be in order.
Now running Jessie!
As a last point. it shouldn’t need saying but remember to back up all valuable documents!!! This would for me include gpg keys, emails, pictures, films, writing, bookmarks (chromium, iceweasel, midori), zotero, passwords and so on. If things had gone to shit (as at a certain point it seemed like it was) at least i still had everything saved on an external hard drive and ready to be reinstalled.