Update README.md

Author: RyanHecht

README.md

@@ -67,7 +67,7 @@ You can also authenticate using a fine-grained PAT with the "Copilot Requests" p
 
 Launch `copilot` in a folder that contains code you want to work with. 
 
-By default, `copilot` utilizes Claude Sonnet 4. We also support GPT-5 via an environment variable. Run `COPILOT_MODEL=gpt-5 copilot` to launch in GPT-5 mode. Or on Windows, run `set COPILOT_MODEL=gpt-5` before running `copilot`.
+By default, `copilot` utilizes Claude Sonnet 4.5. Run the `/model` slash command to choose from other available models, including Claude Sonnet 4 and GPT-5
 
 Each time you submit a prompt to GitHub Copilot CLI, your monthly quota of premium requests is reduced by one. For information about premium requests, see [About premium requests](https://docs.github.com/copilot/managing-copilot/monitoring-usage-and-entitlements/about-premium-requests).