Port Coquitlam, BC
You don't modify the nameservers in Company A's cpanel.
You don't need company A's cpanel if you are going to use Company B for hosting.
Company A should have a portal (or if they are not modern you may need to open a support ticket) to change the nameservers at the registrar level and not the hosting level.
Hope this helps.
PS: who is company A if you don't mind sharing we can provide better instructions for you most likely.