changing date and time on photo file itself

I want to organise my photo library so that they all appear in the library (more or less) at the time that they were taken.

I have a lot of photos and duplicates and photos with wrong capture date for different reasons: received by whatsapp and whatnot, old analog photos that were scanned recently or simply because someone didn't adjust the date and time on their camera prior to taking photos.

I haven't decided which photo software to use for good thus trying to change the date and time on the file itself. Finder doesn't care about EXIF-Meta-Data so I tried to use Mac OSX Photos to organise the photos and adjust date and time, find duplicates and delete photos. However, it's harder than I thought it would be, I can't just drag the photos from the library to a seperate folder on my machine and use this folder to fill the library. The EXIF data seems to be saved somewhere else in the macos photos library? 

I hope this is understandable and I appreciate all advice.