docs: add upgrade instructions

Update installation instructions to include upgrade command.

Author: kevinsbarnard

README.md

@@ -16,10 +16,10 @@ Authors: Kevin Barnard ([[email protected]](mailto:[email protected])), Paul R
 > [!NOTE]
 > VARS GridView requires Python 3.8 or later.
 
-To install VARS GridView, run:
+To install or upgrade VARS GridView, run:
 
 ```bash
-pip install vars-gridview
+pip install --upgrade vars-gridview
 ```
 
 ### From source