At this time you can not edit the metadata for your images directly in the block editor for the WordPress app. You can edit the image block settings, such as Size and Alt Text, by clicking on the gear icon in the lower right corner of the block.
You can edit the image metadata from the media library of your device. To do this go to My Site > Media and select the image you want to change the metadata for. Swipe down to the bottom of the page to see the metadata.