STEP 1:

Visit the 1&1 admin area by clicking here.

Log in here using the details you entered when buying the domain initially.

STEP 2:

Once logged in, click on the menu item 'Manage Domains'. (see highlighted item in image)

STEP 3:

Tick the box tot he left of the domain you wish to modify

With this ticked, click the button from the options above;

'Set Nameservers'.

STEP 4:

Finally, from the right hand side menu (see image) change the option below the white box to: 'Custom name servers' .

Under the settings below, enter the server names:

Name Server 1 : ns1.solarhost.co.uk
Name Server 2 : ns2.solarhost.co.uk

NOW CLICK SAVE