I'm not sure you can, but maybe there's a regkey change to allow it. Your best bet is to get ahold of Support to see 1. If this service port can be changed, & 2. The supported process to do so.
To change the port for the Veeam Gateway API, you will need to contact Veeam support for assistance. The process involves stopping the conflicting service, updating the configurations for any URLs, and then changing the port in IIS. This is a complex task and may require professional support to ensure compatibility and proper configuration.
@hs08 - Just wanted to follow up to see if you received a response from Support to answer your question. Please mark a best answer if this is resolved including your own if needed.