Summary.

1) Enter t your Wix Account, filling in your username and password.
2) Go to "Edit Website," with the blue button on the top/right.
3) When you are on the editor, find the list of web pages.
4) Select the page that is containing the galleries.
5) Once on the page, select the gallery that you like to manage and update photos.

Stept for Update Photos.

Wix galleries on your website work through an internal files section.

1) Select the gallery and immediately appears some icons (option).
2) Select "Manage Media" Options.
3) Next, you have to upload all images that you like to put on your gallery. These images come from your computer.
4) On the File Management framework that belongs to Wix Gallery, you need to select the Upload option, orange button. You can Upload Images or videos.
5) Next, you will be on your website's central files manager. Here you can save your images and videos; my suggestion is to create a folder by a gallery. 
6) Select photos or videos from your computer and upload them.
7) Select all images uploaded by you that you like to put inside your gallery, and press ok.
8) Next, all your images will appear on your gallery media wireframe. Press save or accept.
9) Now your gallery should be updated.
10) Remember always before whatever works on your galleries, website etc, press "PUBLISH", if you like to publish all changes.