Like all the Stacks settings, the Alt Tag field appears in the righthand sidebar.
You can open the sidebar with the “i” button on the image or the “i” button in the toolbar at the top of the window.
If your screen is small, try closing the Library first. If you have a larger screen make sure to open the window nice and wide so you can show the library and the settings.
To open the image’s settings (or any other type of content) you can click the blue Edit button or double click the stack. Once you’re done you can click the Done button or simply click anywhere else in the window.
Here’s a quick gif to show how it works:
