Facebook app confusion!

How do I change the initial Facebook app on my Iph 6splus iOS 12.2 to my new Facebook app account? Eg. When sharing something from Safari my Iphone states that it is 'logged into' my initial Facebook account. I want to keep that initial Facebook account but not on my Iphone, so how do I delete it off my iphone and replace it with my new Facebook account? I've tried downloading the Friendly app but that still doesn't change anything! Any assistance greatly appreciated.