Media details include the title, caption, alt text (for images), and description. At this time, you can only edit the alt text directly in the block editor for the WordPress app.
You can also edit Image block settings, such as the style of the image, size, and link settings by clicking on the gear icon in the lower right corner of the block.
You can, however, edit media details from the media library of your device. To do this go to My Site > Media and select the image you want to change the media details for.