Here's how to disconnect your Facebook account from your Instagram account:
Using the Instagram App:
Open the Instagram app and make sure you're logged into your account.
Tap on your profile picture in the bottom right corner to go to your profile page.
Tap the three horizontal lines in the top right corner to open the menu.
Tap on "Settings" (gear icon on Android, Settings on iPhone).
Scroll down and tap on "Accounts Center".
Tap on "Accounts" to see the accounts linked to your Instagram account.
Find your Facebook account and tap on it.
Tap "Remove" next to your Facebook account.
Confirm by tapping "Remove" again.
Using the Facebook App:
Open the Facebook app and make sure you're logged into your account.
Tap on the menu icon (three horizontal lines) on the bottom right corner (Android) or top right corner (iPhone).
Scroll down and tap on "Settings & Privacy".
Tap on "Settings".
Tap on "Accounts" to see the accounts linked to your Facebook account.
Find your Instagram account and tap on it.
Tap "Remove" next to your Instagram account.
Things to Keep in Mind:
I hope this helps!