Of course yes! You can edit your contest at any time. Once your contest has received one or more entries, you won’t be able to reduce the prize money on offer (you can increase it anytime though!). And, as you’d expect, you can’t edit a contest that’s ended.
To change the brief of your contest, you need to go to your contest page and under the "Design Brief" tab you will be able to edit the brief. Learn more about changing the title of your contest.