1. step:
Go to Customer Portal, via the Web Hosting Settings.
2. step:
On the left, select Upgrade Web Honsting Plan, then click the Modify button next to your hosting ID.
3. step:
Then you just need to choose which hosting plan you want to switch to. The system will calculate the exact amount you will have to pay, which will be a pro-rated differential.
To finalise the switch, you will need to select the payment method and accept the terms and conditions of the switch.