Depending on what packages you've got installed and what property editors you currently use. It should be a case of porting your existing code to a new Umbraco 10 codebase (latest version). Obviously there's API changes (e.g. RenderMvcController is now RenderController). You then point your new v10 codebase towards your v8 database (remember to back up!) and let Umbraco update your database schema. All being well everything should be as it was before. However, as I said before this all depends on what you've got on the go. If you use packages which weren't ported to v9+ then you may need to figure out a workaround. Once upgraded to v10, you can then proceed to v13 (LTS to LTS). I hope this helps, I'm on my phone at the moment and don't have all my bookmarks, but there's many blog articles which help share personal insights, as well as the Umbraco documentation itself.

Umbraco has a guide for that: https://docs.umbraco.com/umbraco-cms/fundamentals/setup/upgrading/version-specific/upgrade-from-8-to-latest

Basically:

• Update your data (the database) to the latest version
• Create a new blank Umbraco 13/14 project, attach your updated database to that and step by step copy over your code and change if necessary.

I would suggest you upgrade to Umbraco 13 first, because in Umbraco 14, things like Nested Content and macro's are removed. This makes upgrading to 14 much harder.

A few hints:

• If you are upgrading Umbraco forms, make sure to update to the latest version of 8 first, otherwise you might get a migration error. Also store the form data in the database: https://docs.umbraco.com/umbraco-forms/developer/forms-in-the-database#enable-storing-forms-definitions-in-the-database.
• When upgrading the database, also include any package in the correct version that adds something (like data editor) to the backoffice, like Contentment, GMaps or Seo1902. I don't think it's strictly required, but to me it feels like this is more reliable than doing if afterwards.
• Most namespaces have changed, but they are not that different
• @html.partial does not exist in .NET core, you need to rewrite those to ViewComponents
• Umbraco 9+ uses ILogger for logging, changing the signature a little.
• If you were not using Dependecy Injection in Umbraco 8, you really should use it when upgrading.
• I can't find the documentation for it anymore, but I'm certain it was recommended somewhere to upgrade to 10 first, then continue to 13.

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