User connects an Instagram Business account; when they do so, that particular Instagram account gets linked to the FB page and every time the Facebook page is used to publish ads, we see the Instagram account automatically selected. Now, considering the fact that user wants to delink this Instagram account from the connected Facebook page and connect it to a different page.
Disconnect your Facebook Page and Instagram account
Log in to Facebook, then click your profile photo in the top right.
Click See all profiles, then select the Page you want to switch into.
Click your Page’s profile picture in the top right of Facebook.
Click Settings & privacy, then click Settings.
Under Permissions, click Linked accounts.
Next to Instagram, click View.
Click Disconnect account, then click Yes, disconnect to confirm.