This list serves as a guideline how to update your Orchard source if you maintain a copy of the full source (see a description for this).

1. Clone or pull the latest source from the main Orchard repository and checkout to the changeset you want to update your instance to.
2. Archive a snapshot of the repo at the specific changeset.
3. Remove the lib folder in your own solution folder. Outdated libs can cause nasty errors.
4. Copy the source over to your own solution folder, without the .gitignore file (unless you use it in your own repo of course).
5. These only apply if you're using a solution file other than the default Orchard.sln (if you're using Orchard.sln just merge that file:
   • Merge Orchard.proj (it references the solution file).
   • Add any new modules to your solution and remove deleted ones (including the removal of the modules' folders).
6. Merge Orchard.Web.csproj. You may have some custom files included there.
7. Merge Orchard.Web/Web.config if you have modified anything in the original Web.config (better to use custom config files for different build targets) or the targeted Web.configs (Release and Debug).
8. Rebuild the solution to check for any build errors.
9. Run the site to test if everything is working as intended.