Can I change main company number in order to port out main company number to a different provider

Yes, it's possible. You must add a company number in the Admin Portal and then swap it with the main number. Ask assistance with Tech Support in swapping it as there's no option in the portal.

Here's a KB article for adding a company number.

https://support.ringcentral.com/article/Add-Company-Number-RingCentral.html