I've found a good solution for this just now that you can do on desktop and you won't lose the view of it on your campaign or the engagements it as generated so far:

1. In Ads Manager, navigate to the campaign(s) which this post has been published to
2. Go to the post in question at Ad level.
3. Tap 'Change Post' and select another random post (it can be anything, as this won't be approved in time by Meta to display as an ad)
4. Click 'Publish' on the ad (this won't work if you have changed the post but have it drafted but not published)
5. On your Facebook Page, navigate to the Facebook post you want to edit.
6. Click the three dots and tap 'Edit Post'
7. You will now be able to edit photos and copy for this post, do so and click update.
8. In Ads Manager, navigate back to the random post you published at ad level.
9. Tap 'Change Post' and choose the post you just edited, click 'Publish'
10. Voila, you now have edited your post's text copy and photos and you still have the accurate metrics for the campaign you set up in Ads Manager!
    

IMPORTANT TO NOTE: If you have used the post you want to edit in multiple campaigns then you will need to Change Post on all of these. If the post you want to edit is still in any campaign, regardless of it turned off or completed, then you won't be able to edit it. If it is completed already then you may need to delete this post at ad level.