Step-by-step guide for upgrading an Optimizely Commerce site from Commerce 14/CMS 12 to Commerce 15/CMS 13 preview, including a .NET upgrade from 8.0 to 10.0. Covers project configuration changes (target framework, NuGet package references), Startup.cs service registration updates from a simple metapackage approach to explicit service registration, appsettings.json changes for Content Graph, and running database migrations. Preview versions are not recommended for production use.