Editing the security group binding for a vNIC will interrupt traffic forwarding on the vNIC and interrupt the network connection on the cloud host that owns the vNIC. Make sure you are fully aware of the impact of this operation on your network.
On the top navigation bar, click Cloud Services.
From the left navigation pane, select Compute & Storage > vNICs.
Select a security group from the Available area and add it to the Selected area. As a best practice, make sure the Selected area contains only one security group. (See "Restrictions and guidelines" for the reason.)