Hi Lara,

i deleted the two large faces. The small one at the top i filleted the
edges. When doing so you need then to use the rounded line it
creates to trim the small surface. Like i said you probably need
to use a larger fillet radius to get it smoother. After that i used
the network command to recreate the two large faces.

I think if you could post the model without fillets people can come
up with a better solution.

The red line was extended by drawing a small line and then joining
both lines.

Regards
Stefan