Currently an internal user cannot update a client's email directly. This is a feature on our roadmap.
There are two workarounds:
- Ask the client to log in and click on Settings > Account. On this page, the client can update their own email address.
- Go to the Clients page and click on the client to navigate to the Client details page. Click on the action button (⋯) and select Reset password. Proceed to reset the password, log in as the client, then follow step 1 above and give your client the temporary password that you generated.