Copy the last part (everything after base URL) of the installer link provided to you, e.g. https://login.salesforce.com/packaging/installPackage.apexp?p0=04tj0000001iMva2 (red text).

Login to your Salesforce instance and remove everything after the base URL, then paste the last part of URL at the end:

Go to resulting page.

The Upgrade dialog will appear. Please select "Install for Admins Only" and then the "Upgrade" button:

After pressing "Upgrade", you will see the following status screen:

Since Work-Relay is a large package, you will see the following notice:

You can click "Done" and you will receive an email when the package update has finished.