Making flat faces rounded

I’m currently working on a model for a gun for a game.

But, as you can see here, the handle for the gun is very flat and square:

So what is the easiest way to make it more curved and rounded? Should I make extra vertices on all the edges and make them round my self? Or should I use a curve or a circle plane and edit it in?

Here is another pic:

Generally, yes, you need to edit it. But to make things easier, there are tools to use in blender. Don’t be affraid of adding some lines if you see you need them. Don’t add too much at once.
While there are several ways of making such object, try this

You can try to bevel round those edges. You can find the addon in addons -> mesh -> extra tools. then select edges You want to make ronded and press W -> exttra tools -> bevel round