This is pretty simple and you can do it in a couple of ways.
1) Firstly, just use the Settings menu:
This will jump to straight to Step 2 of the Campaign Setup Wizard (see below).
2) Alternatively, go to the My Sites section:
and then edit the Campaign - click on the Actions column to see this:
Click on Edit Campaign - another blue icon:
Go to Step 2 of the Campaign Setup Wizard and change the competitors by either simply editing them in the boxes on the right (red arrow) or by search for new ones using the facility on the left hand side of screen (purple arrow) to find competitors by using a keyword:
Remember you need to follow the wizard to the end by clicking on the green Next buttons and clicking on Save at the end of the wizard in Step 5!
NB: the platform supports unlimited campaigns for each domain, so an alternative to editing competitors is simply to add on an additional campaign and add on another 1-5 competitors. Once reason why you may want to do this is that you can then setup one campaign for small and local competitors for a domain, e.g., and another for larger, national competitors. If you do this, the platform will provide competitive backlink data for both sets of competitors. This gives you a much wider range of link prospects for use in your own link-building efforts. Looking at the backlink profiles of different competitors might well give you different ideas as to best approach your own link-building campaign. You will also get more data from the Competitive Site Audit component and we also obtain more data for the Competitive Keyword Ranking component (please note, though, that for this to work you would need to copy your keywords across to the new campaign or use some additional keywords in subsequent campaigns and this may affect your account usage).