Can I upgrade a User on a Sub-Account to an Admin on the Parent account?

Yes, you can.

Go to Account and then Users, and alongside the user you wish to upgrade, click on the ellipses (three dots) and select Edit profile:

In the following screen, look at the Account and sub account access section, where you will see the accounts and sub-accounts to which the user has access:

Click on the down arrow next to the Advanced option in the account for which you want to amend the user's role, and then select the required role from the resulting drop-down menu:

(You can also remove a user's access to specific accounts or sub accounts by clicking on Remove role on the right hand side of the relevant account item)