1) Login to your client area at https://portal.ajrbcloud.uk
2) Select 'Domains' on the left pane
3) Select the ellipses of the domain you wish to change the nameservers for, and then click 'Manage Nameservers'
4) Click 'Use custom nameservers' and enter the desired nameservers
Click 'Change Nameservers'
Note that these changes will be applied instantly to the domain registry, but DNS by design may take 24-48 hours to propagate globally (although these changes typically take effect within a few minutes).