How to connect your Facebook page and Instagram account
Go to Your Pages.
Locate the page you want to switch into.
Click Manage Page.
Can’t find “Manage Page”? Click the three dots next to your page and select Switch Now.
Once switched to your Facebook Page, in the left pane you’ll see options like Professional dashboard, Ad Center, and Settings.
Click Settings.
Scroll down in the left pane, and under the Permissions section, click Linked accounts.
Next to Instagram, click View.
Click Connect account, then follow the steps to connect your Facebook Page and Instagram account.
You can read more here.