Well partial removal, I was looking up for this, since my avatar had an annoying grain, even though this tutorial will not completely remove it, it atleast reduces it to a certain extent.
Example: ( Look at the Hair Area )
What this will do: ( Look at the Hair Area now, its more smoother )
As you can see, this will partially remove the Grain Effect, or cover it up as I should say.
▼ Tutorial Below ▼
If you want to do the whole image at once, what you do is go to: Blur > Gaussian blur
But if you don't and only want a specific selection of the image.
Then you go to Rectangle Select Tool or a Selection tool then go to: Blur > Selective Gaussian blur