How can i edit my integration so that the form submissions saves in a subfolder that already exists?

You can edit your Dropbox integration by clicking on Edit Integration button. 

Once you click on the Edit Integration option you will be able to change the name of the subfolder where the submissions are sent. However, please note that this change will be applied only for your new submissions. The old submissions will remain in your old subfolder. To push the old submissions to your new folder you will have to edit your previous submissions and resubmit them once you edit the integration. 

How to Edit Submissions Made on Your Forms

You do not have to edit any data in your previous submissions. Simply click on the edit option and submit the form. 

That's what I did and it didn't work basically here is the structure of my dropbox

Dropbox > Folder A > Folder B > Folder C

And I want the submission to be saved under subfolder C and it doesn't work. It just creates a new folder within dropbox so at the same level as Folder A.

As far as I know, Dropbox integration cannot have 2 subfolders. You can only have 1 main folder (Folder A) and one subfolder (Folder B).