To change the billing cycle for your Ecommerce package, please do the following:
Log in to your 123-reg control panel.
Scroll down to the Ecommerce section.
Click on the Manage ecommerce plans option.
If you have more than one ecommerce package, click on the Manage button for the relevant package.
Click on the Change link next to the I want to be billed: option.
Select the desired billing cycle from the drop down box and click on the change button.