Select adjacent faces of the edge to be rounded or alternatively the edge itself.

Cube: Adjacent faces of edge to be edited selected

Call the context menu command New Chamfer/Fillet....

-> The dialog box Chamfer/Fillet is opened.

Select the Fillet option and in the input field, enter the desired Radius.

Dialog box Chamfer/Fillet: Here exemplarily with option "Fillet" and Radius = 3

Example in 2D: Rounding with a diameter of 3 at a length of edge of 10.

Confirm with OK.

-> The rounding is inserted.

In 3D History, on the blend feature, by clicking on Edit..., you can call the dialog anytime.

You can adjust values subsequently or switch from Fillet to Chamfer.

Switching from Fillet to Chamfer

Example in 2D: Chamfer with setting "Distance 1" =3 and "Distance 2" = 1 at a cube with edge length = 10

Chamfer in 3D view