You almost certainly can't.
STL files contain many triangular facets. Going from 'proper CAD' to STL is easy, but detail is inevitably lost. A straight conversion from STL to CAD will replicate all those triangles and give a huge file size. You need specialist software to reverse engineer the original geometry (try to work out where the curves were).
You could try deleting faces one at a time using the direct editing tools - it may allow you to get rid of some facets and fill in the gaps. If it works it will be slow, and it may not work.