Or: "How to cleanup the mess..."
A couple of days ago a friend that is new to Magento reached out to me asking for some advice on how to update a Magento instance. He had inherited a legacy Magento project from another friend and now the merchant wants him to update the shop to a recent version of Magento.
While there are a couple of posts available on how to update Magento none of them covers how to deal with a really messy project in the first place and how to update that one in a safe way. Let my try to explain my approach.