If the web address starts with

https://arribada.ebay.co.uk (note the https ) should be a padlock in the bottom pane of IE also..... then you should be okay. - it is one of ebay's own addresses.

Why are you updating it - did you get an email asking you to, or just doing it because you want to?

Whenever changing your details in ebay, ALWAYS type in www.ebay.co.uk in a new browser window and log in from there, don't follow links in emails.....