How can I smooth the edges around this shape (sketched and scanned to STL)?

https://preview.redd.it/x1sum2ldr2vf1.png?width=1583&format=png&auto=webp&s=39ddd37871f49bbb941f2f9bd1620020ed82bc5c

Hi there - I am trying to create a little water tray for my fridge -- so I cut out a piece of paper to the shape of the opening and scanned that in. I'm not sure what the best way is to smooth these edges, anyone have any advice?