Hello @Kiranmai,
To get any app, use the following command:
bench get-app --branch git-branch git_app_path
// Example
bench get-app --branch version-15 https://github.com/frappe/erpnext.git
For app installation on a specific site, use:
bench --site your_sitename.com install-app erpnext
To update the erpnext app, navigate to the app directory and execute:
git pull upstream version-15
Ensure that you set the version as per the app requirements.
Here are some references for your guidance:
I hope this information proves helpful.
Thank You!