How To Edit Business Description For Google My Business
Google Business Description Feature Removed
You can no longer edit your Google Business Introduction/Description field via Google My Business portal. It can only be edited via your Google+ profile.
Google removed the feature 1 August 2016, and had the following to say:
The Introduction/Description field is no longer editable in Google My Business. It only displays to users in Google+, and may still be edited there. Editing of attributes, coming soon to all Google My Business views, will be an improved way to describe your business to users on Google Search and Maps.
The Introduction/Description feature was removed due to Webmasters abusing it with spam keywords.
To edit your profile navigate to aboutme.google.com (you must be signed into your gmail account)