To switch your Instagram to a Business Account through Facebook, first go to your Facebook Page, then scroll down on the left and click Settings
Then scroll down on the left and click on Instagram

Then click on a button where it says Switch Now

If you do not see a Switch Now button then you will probably have to switch to a business account using your mobile Instagram app. To do so, you can follow the instructions from Instagram here: https://www.facebook.com/help/instagram/502981923235522