STEP 1:

Visit the 1&1 admin area by clicking on http://admin.oneandone.co.uk

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

STEP 2:

Once logged in, click on the logo for the 'Domain' area of the control center. (see circled item in image)

STEP 3:

Tick the box tot he left of the domain you wish to modify. With this ticked, click the small arrow next to the DNS button. From this drop-down menu, select 'Edit DNS Settings'.

STEP 4:

Finally, from the drop-down menu (see image) change the server from 1&1 nameserver, to 'My name server' .

Under the advanced settings below, enter the server names:

Primary: ns1.solarhost.co.uk
Secondary: ns2.solarhost.co.uk

NOW CLICK SAVE