how do I change my author photo?

Avatars in WordPress come from Gravatar with the need for a plugin – they just work with Gravatar as part of core WordPress. If you update your avatar on Gravatar then it will update on your site, but it may take a little while for the cache to be cleared and your avatar to actually change.

I know you said no plugins, but the easiest way to use a custom avatar image is by using a plugin like this one: https://www.ads-software.com/plugins/wp-user-avatar/

If you’re comfortable with a bit of code, you can filter the avatar display by using the get_avatar filter: https://developer.www.ads-software.com/reference/hooks/get_avatar/