docs: add instructions for syncing fork/copy with Tier IV AWSIM

docs/DeveloperGuide/ForkRepository/index.md

@@ -38,6 +38,61 @@ The following are the recommended operational practices when AWSIM is forked or
      │  │ 
      ┊︎  ┊︎  
     ```
+
+## Updating from Tier IV AWSIM
+
+Whether you **fork** or **copy** the AWSIM repository, the workflow for keeping your custom AWSIM up to date with the original Tier IV AWSIM is essentially the same.
+
+- **Fork**:  
+  Clicking the Fork button on GitHub creates a replica under your account (`origin`). However, when cloning locally, only `origin` is configured by default — no `upstream` remote is set automatically.  
+
+- **Copy (private repository)**:  
+  Creating a private repository and pushing the original AWSIM code into it results in the same situation: only `origin` is configured.  
+
+In both cases, you must **manually add `upstream`** to pull updates from Tier IV AWSIM.
+
+1. Set upstream
+In your custom AWSIM repository, run:
+```bash
+git remote add upstream [email protected]:tier4/AWSIM.git
+```
+1. Fetch and merge updates
+
+    Fetch the latest changes from the Tier IV repository:
+    ```bash
+    git fetch upstream
+    ```
+
+1. Merge or rebase them into your local main branch:
+    ```bash
+    git merge upstream/main
+    ```
+
+
+1. Push the updated branch to your own repository (fork or private copy):
+    ```bash
+    git push origin main
+    ```
+!!! info
+    Notes on pull / push
+
+    By default, git pull and git push only operate on origin (your fork or private repository).
+
+    To retrieve updates from Tier IV AWSIM, you must explicitly specify upstream:
+    ```bash
+    git pull upstream main
+    ```
+
+    A normal push:
+    ```bash
+    git push
+    ```
+
+    will only update your repository (origin), never the Tier IV AWSIM repository.
+
+!!! tip
+    Fork and Copy behave the same when synchronizing with Tier IV AWSIM:   
+    Add upstream → fetch → merge → push to your own repository.
 
 ## Namespace