Hi Nabil

I don't think there's an option to change just one edge of a rectangle. However if you go to the top toolbar under "Shape Options" you can adjust to make the rectangle more or less rounded along the edges. The lower the "px" the less rounded the shape is. 

Hey guys! 

Thanks so much for posting in the Lucid Community and being so willing to help one another! Just a heads up -- you may want to check out our Flex Polygon functionality. You can adjust these to whatever you would like and then save them as a custom shape in your library! 

Hope this helps -- please let me know if you have any other questions. Cheers