add theme update instructions

README.md

@@ -52,3 +52,12 @@ Delete the old branch:
 ```sh
 git branch -d "$(date +%F)-update"
 ```
+
+## How to update the theme
+
+You can update a theme to the latest version by executing the following command in the root directory of your project:
+
+```
+git submodule update --rebase --remote
+```
+