...where we will find the field on the Fields and Relationships section of the Object Manager/Donation and Object Manager/Campaign

I put "Donation" in the search box, because we've renamed the "Opportunity" object "Donation" which is more in line with how most nonprofits thing about the revenue that comes into the system.

Here's the field. It's a Formula that looks to the Campaign object. Not sure which one we can delete first. Try this one. If that doesn't work let's go to the Campaign object and find the field there.  

Click the Object Manager again and this time, enter Campaign in the Quick Find box. This is easier than scrolling down.

I didn't show the Quick Find box this time because you found it with Donations, so it hasn't moved...you can find that again! Click on the Campaign and then, from the Details column on the left side, click Fields and Relationships.

To delete a field, find the little down-arrow at the end of the row.

This one is a straight-up picklist, not a formula. My guess is that you have to delete the one of the Donation object first, because it is going to be looking for the campaign, and won't want to go away with that dependency intact...but this is one of those spots where I would just try this one, and if you get an error message, try the other one.

If the setup between these two fields is preventing you from deleting either one of them first, let me know. I'll edit the Donation field so it is NOT dependent on the campaign, and that should be all we need to get this done. But I'm hoping this is all you'll need.

GOOD LUCK!

By the way, you might want to use this in the future to find fields that you think have already been created but you're not seeing them, or you want to edit a field, for example, to add more space for a longer description or you want to add a value to a picklist. This is where all that good stuff lives.