Here's how to blur the background
in Photoshop CC.
Go to "File," "Open," and open your image.
Go to the toolbar and select the blur tool.
In the options bar, choose the brush tip
you decide to use for blurring
and adjust options for the blending mode
and strength of the blur effect to your liking.
You can also choose to click "Sample All Layers"
in the options bar to blur using data
from all visible layers
when you're working with composite images.
Now you can drag the blur tool
over any part of the image you wish to blur.
You can also choose to use more advanced blur filters
found in the filter tab at the top of the screen,
in the "Blur" and "Blur Gallery."
You can selectively control what's blurred
through the use of any of the selection tools
found in the tools window, including quick mask mode,
which gives you the ultimate control.
Simply press the Q key on the keyboard
to enable quick mask mode.
This will help you identify
and remedy any uncaptured pixels.
Areas that have been captured
as part of the background will be red.
Zoom in so you can spot the edges and details
that may not have been automatically captured.
Then select the brush tool on the left toolbar.
Paint in pixels as you see fit
to be included or not included in the background.
To include pixels in the background,
paint them black with the brush tool.
They should appear red in quick mask mode.
To remove pixels from the background,
paint them in white with the brush tool,
and they'll be added to the subject.
When you're finished,
zoom out and tap "Q" again
to exit quick mask mode.
Now you're free to use any of the blur filters
found in the blur tab underneath the filter tab
found at the top of the screen.
Simply choose a blur filter that fits your needs,
adjust the parameters to your liking,
and apply to the selected background area.
Deselect the selected area by pressing command + D
on your keyboard for the shortcut.