To do that, go to SEOPress main menu.
Check the green radio button associated with Robots.
Then, click Manage, and check Enable robots.txt virtual file.
In Virtual Robots.txt file textarea, create your robots.txt and click Save changes.
To view your robots.txt file, click the button View your robots.txt, or go to http://yoursite.com/robots.txt
If you see a 404 error, flush your permalinks.
Notice that, if your have a physical robots.txt file, this virtual robots.txt file will not bypassed it.
Some servers automatically add a physical robots.txt file that can only be surpassed by creating your own physical robots.txt file. In this case, the virtual robots.txt of WP or SEOPress can not be used (case with some web hosts like Infomaniak).