I have a business and had Google verified business listings, There are negative reviews given by some online competitors, not from my customers. I have a 2.8 rating over there so I want to remove those reviews OR I want to improve my review rating at GMB.  How?

You can not those remove while you can communicate with reviewer to change their review and try to know reason for their hate and negative reviews, you can also flag them in the GMB section and request to Google to remove these reviews.

Follow these simple steps to dispute a Google review:

Visit your Google My Business profile
Click on the "support" option in the navigation panel
Select "Need more help"
Click "Customer reviews and photos"
Select "Manage customer reviews"
Then click "Email support"