For now, this document will only list updates from the past two years, but it will eventually be improved to go further back.
Most of the time you will be able to upgrade directly from your current version to the latest version. Follow the steps in the general upgrade guide, then these additional instructions for the specific versions. (Remember that new config files are not mentioned because they are already covered in the general upgrade guide.)
Read and follow the full v7 upgrade guide
There was an update to the /umbraco/config/create/ui.xml which needs to be manually updated, the original element had this text:
<create assembly="umbraco" type="userTasks" />
<delete assembly="umbraco" type="userTasks" />
Make sure to manually clear your cookies after updating all the files, otherwise you might an error relating to Umbraco.Core.Security.UmbracoBackOfficeIdentity.AddUserDataClaims(). The error looks like: Value cannot be null. Parameter name: value.
Value cannot be null. Parameter name: value
NuGet will do the following for you but if you're upgrading manually:
<add key="owin:appStartup" value="UmbracoDefaultOwinStartup" />
For manual upgrades:
Invalid object name 'umbracoUser'