How to correct / change reference to a picture in photos

Hi Leonie - Tks a lot for answering sa fast 🙂 - No, my problem is that the reference from the thumbnail (which is correct) to the file on my mac is wrong - i.e. I have to "fix" the reference... did I make myself clear ?

..so right now the thumbnail is the correct picture, but it references to a wrong picture file on my computer (note: my preference is configured to not copy all into my library, I leave all on separate folders on my mac)..

So I upgraded to Photos from iPhoto some 6 months ago (upgrading to yosemite) - and let's say I want to start from scratch - first question, pls focus on the 4 options on top of Photos initial window: "Photos", "shared", "Albums" and "Projects" - so to begin with I see that the 'Photos' section is a mess, dates wrong, several duplicates of same - so let's say I want to erase all pictures and videos in "photos" section, BUT.. I want to keep the "Albums" section - how do I do that?

Leoni - So there is not a way to just close your eyes and erase the entire Photos Library (except the already made albums) and just let Photos restart importing thumbnails from my entire picture & video folders? - Another questions - is it correct that the "Photos" option is an unorganized look at all the pictures and videos on my mac? - or is the "Photos" option actually configured to only create thumbnails from the folders I ask Photos for Mac to build its library from for the "Photos" option?

PS: yes I still have all the original pictures and videos in my folders outside photos library - but we are talking abt several hundred or maybe thousands which are wrongly referenced... so my wish is to just re-ignite the Photos library.. but again, I do not want to re-make all my albums again?

Ok.. all problems started upon upgrading to new Photos from iPhoto - I can see that inside the "Photos" option, several a referenced wrongly - the "Albums" are ok, as they automatically 'copied' the pictures and videos into the album library - but the "Photos" option is a big mess - so I would like to just erase every single picture and video from the "photos album and just re populate it again automatically directing the "photos" search to the specific picture and video folders on my mac - is this possible?

I have an idea - I apparently can still open the iPhoto library, using the iPhoto app (version 9.6.1) and in this app I can see that all the references are ok and working - also the albums are perfectly present - can I erase or eliminate the Photo library, then reboot mac and then open Photo, thus forcing it to create a new photo.library and hope all will be migrated perfectly?

Tks for your feedback - so what is my suggested alternative? - and does this mean I should start from zero?

Tks - so, if I do not need to erase the photos.library, then how to i create a new version of the photos.library, if the present one is not to my liking - i.e. too many reference errors?