Only a main contact can make a secondary contact a main contact. Click here to see what a main contact and secondary contacts can do.

Here's how to update the main contact.

In the Child Profile Page, go to the Contacts Tab. Find the contact that you want to make the main contact. Click on the Main Contact switch. Answer the questions in the ensuing dialog box. 

A pop-up will open asking whether you want to continue as all billing, ledger will transfer to the new primary contact and update the payer also. If you don't want to transfer these rights, you can uncheck the check box and click continue otherwise just click continue.

Then this new contact will became the primary contact.