Tutorials: How to change nameservers - One and One
In order to change your 1&1 nameservers you'll have to :
Login to your 1&1 account
Click on "Manage Your Domains"
Check the domain
At the top of the list, there is an arrow next to DNS settings, click it and clicl "Edit DNS Settings"
A drop down box will appear with : "My Name Server" as an option
Fill in the nameservers from the welcome email
You're done