The Round Corners effect converts
the corner points of a vector object to smooth curves.
In the Layers panel, target the items you want
to round.
If you want to round a specific attribute of an object,
such as its fill or stroke, target the object in the Layers panel
and then select the attribute in the Appearance panel.
Choose Effect > Stylize > Round
Corners. (The command is located in the first Stylize submenu.)
To define the curvature of the rounded curve, enter a
value in the Radius text box, and click OK.
You can achieve the same results with the
Round Corners filter (Filter > Stylize > Round
Corners). However, you can’t change a filter after you apply it,
whereas you can modify or remove the effect at any time.