I'm using Exim and CPanel. My ISP blocks outbound smtp port 25. Is it possible to change the default Exim outbound port to 587 using CPanel?

Yes, this is possible. Here are the steps:

WHM > Service Configuration > Service Manager, you'll find an area there called "Exim On Another Port".

For example, you could use port 587 here. Tick both boxes there and save. You'll also need to open that port in your firewall and explain to your users how to change this in their email clients.