doc: Document how to update

Author: nullpo-head

README.md

@@ -98,6 +98,7 @@ By this installation, systemd is enabled in your WSL 2 distro.
 
    ```bash
    curl -L -O "https://raw.githubusercontent.com/nullpo-head/wsl-distrod/main/install.sh"
+   chmod +x install.sh
    sudo ./install.sh install
    ```
 
@@ -152,6 +153,16 @@ For reference, the following command launches a distro by name in WSL
 > wsl --distribution Distrod
 ```
 
+## Update Distrod
+
+1. Inside a Distrod session, download and run the latest installer script.
+
+   ```bash
+   curl -L -O "https://raw.githubusercontent.com/nullpo-head/wsl-distrod/main/install.sh"
+   chmod +x install.sh
+   sudo ./install.sh update
+   ```
+
 ## How Distrod Works
 
 In a nutshell, Distrod is a binary that creates a simple container that runs systemd as an init process,