In this episode, I will explain all the steps that you will need to follow in order to upgrade your website to use Umbraco v8. Before attempting to upgrade anything I recommend that you read the Umbraco upgrade notes, https://our.umbraco.com/download/releases/800
As Umbraco uses .NET 4.7.2 if you use Visual Studio 2017 you will want to make sure you patch to the latest version and ensure you have the correct version of .NET installed.
Before you upgrade to v8 you will need to upgrade your site to at least v7.12. If you do not do this then Umbraco will fail to upgrade your database correctly. When upgrading to 7.12 you may be tempted to skip thorough testing. If the website does not upgrade correctly then the upgrade to 8 will also fail. Some of the data-types change so you need to make sure that your document types do not use any obsolete property types.
After doing all of this you are ready to upgrade the Umbraco.Core and Umbraco.CMS Nuget packages to 8. When you do this it is very likely the next time you compile the site you will get hundreds of thousands of error. GO through your codebase and comment out anything that complains. Keep doing this until the site compiles. Now run the v8 upgrade wizard.
If everything goes to plan, Umbraco will upgrade your database. Now you will need to un-comment all your custom code one line at a time until the website works again.