Select a face whose adjoining edges shall be rounded.

New Chamfer/Fillet...

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 = 2

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

Confirm with OK.

-> The rounding is inserted.

Example: Edge length of 20 and fillet radius of 2

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

You can adjust values subsequently and 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

Example with chamfer