Upgrading Sitecore is easy! Upgrading custom developed code in the Sitecore can be more challenging.
Sitecore is well known for being "easy to upgrade". This is absolutely true for its base code. Sitecore comes with 2 packages for upgrading - 1 for Database and 1 for Code. Using the built-in wizards, you can perform changes to the configuration files and *presto* you are ready to go with a new version.
Easy enough. Especially if you did not build custom external components.