The portfolio snippet block allows you to make changes to all the images and applied captions via a EDIT SETTINGS button that appears on the snippet block.
With the settings popup you can change the following settings across all images
For more fine-grained control of the captions formatting, click the Link Image and Settings (RED CHAIN icon when hovering over the image) > Caption Settings area or the Style Editor (RED COG Icon on the row snippets) popup.
Caption text can also be highlighted and changed via the Text toolbar options that appear at the top. Any changes made will supercede the settings shown in the portfolio popup.