Hi Olga - normally you would use fillet, but it will probably have difficulties along a boundary like that where the curvature flips direction from kind of concave to convex.

The other tool for making things smooth is blend.

To use blend in this situation, you first need to open up some space - I did this by drawing in one line and then mirroring it like so:



Then select your surfaces, run Edit/Trim and select the lines as the cutting objects. Pick the inside pieces as the parts to discard, that will end up with this:



Now select a matching pair of edges and run Construct/Blend to make a smooth piece between them. Repeat with the lower edges and then you can use Edit/Join to glue the pieces together:



- Michael