How to make rounded edges of a face I want to make these 2 inner faces with rounded edges like the left image

At the right image I used Ctrl+B to make the outer edges of this board rounded but I can't apply this to  inner face as the board itself is a cube with some modification
For your example this could do the job:


Enable LoopTools addon in the User Preferences (CtrlAltU > Addons)
Select the outer vertices
Apply Relax function with Linear Interpolation