There are two ways to link your Facebook page to your Instagram account. You can use either Facebook or Instagram. Moreover, the same steps will help you disconnect or unlink a Facebook page from an Instagram account. These are up to date than what is provided on their official website.
Using Facebook
Step 1: Switch from your personal profile to your Facebook page
Step 2: Use the ⚙️ icon to go to Settings
Step 3: Tap on Linked accounts
Step 4: Choose Instagram, and link your account
Using Instagram
Step 1: Tap on your profile picture at the bottom right
Step 2: Tap on 
Step 3: Tap on ⚙️Settings and privacy
Step 4: Tap on Accounts Center
Step 5: Tap on Accounts
Step 6: Tap on Add Accounts, then connect your Facebook page
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