To edit an email forwarding rule that you have previously set up, you first need to log into your 123-reg control panel, then follow the instructions below:
Scroll down the page until you reach the Email section.
Click on the Manage email link.
Select the relevant domain for your email from the domain drop down menu.
For the relevant Forwarder, select Forwarder Settings from the More menu.
This will take you to your Email Forwarder's settings page.