Do you mean rounding the corners of the 6x6 square (radius maybe 2")? Router w/ template and pattern bit. I'd probably rough-cut them with a bandsaw, then use the router template for final shape.
Also, rounding an edge makes it more durable because it won't show dents, splinter, or lose its finish as easily as a sharp corner. Like a knife that loses its sharp edge, a rounded corner is more "friendly" to people or objects that come in contact with it.
Rounding the edges of your design could give it a sleeker, more professional appearance. You can create any size of rounded edge you need, by using a wood router with the right router bit. Round the edges just enough to take off sharp corners or give it a more dramatic look.
Bit more info would be good, no use speculating, how it is rounded and how long the 2X4 is would help. For example rounding the end like a popsicle stick with a bandsaw on a 14' long 2x4 takes a lot of room, 2' long not so much.
no the width. the 2x4 is about 3 feet long but I need just that end to be rounded. i will add dimensions to the image. this is an image of the side of the 2x4.
SPF 2x4's are very soft and for that reason I don't think they would make a very good top for a work bench. If you wish to use 2x4's I would at least try to find yellow pine 2x4's. As far as taking the round edge off the 2x4's I would only do that with a jointer.
Wood routers allow you to round off edges of wood to reduce their sharpness. Keep wood corners safe from injury with help from a professional remodeler in this free video.
rounding the end of 2x4 Cut 2x4 frames and legs How to Build a Cornhole Game Set This Saw the 2x4 side and end frame pieces and legs to length according to the cut list.