Click the right mouse button once (CONTROL-click on the Mac). The selection tool appears (looks sort of like a spindle). Click outside of the bottle, e.g. to the right side of its bottom. A read rubber-band line appears at the first point.

 

Move the selection cursor around the bottom edges of the bottle, clicking here or there to make a selection all around it. This is called "throw the lasso around the desired vertices."

When done, hit ENTER to complete the selection.

Click the Filleting (Chamfert) tool in the middle of the modeling toolkit, or select Tools > Modeling > Fillet. A red preview of the filleting effect appears. Use the +/- keys to change the radius.

    

Hit ENTER when you have the right radius.

  

Use the Arrow keys to rotate down and look at the bottom of the bottle from underneath, and hit 0 to zoom back to entire scene, then hit ENTER to render the scene.

  

Hit CTRL-S to save the current design.