Hi

Whether this can be done depends on:

Sufficient difference between the background and the subject you want to remain in focus - this determines whether it is possible to create a mask from the video.

How accurate you want the background blurring around the subject.

Without seeing the source you are starting with - a screen grab would give us a better idea - below are two examples

A crude example of what can be done - using a simple oval mask in this case - if you do not want close cropping to the subject.

The above video is a slight variation on this tutorial

A good example of what can be done - if there is sufficient differentiation between the background and the subject to create a good mask - in this instance the sky has been replaced.

Just in case anybody thinks those are UFO's in the video - they are not the video clip is a fake and was created in Movie Edit Pro 2016 

HTH

John EB