Hi - one method is to use a boolean operation to carve away a portion of your shape, leaving a curved top behind.

For example, say you have a box with a flat top like this:



You can now switch to the Front view and draw in some curves arranged like so:



Now it is possible to cut the box by those curves - to do this, select the box, then run Construct / Boolean / Diff, and select the curves as the cutting objects. That will create this result:



That has cut the box up into some pieces, if you remove the top and bottom pieces, you will be left with an object with a curved top:



So one way to get a curved top is to use a curved cutting object to slice up the shape.

- Michael