Using an actual desktop browser (rather than a mobile device), from one of the My eBay pages you should be able to reach your eBay profile page by clicking on your username. From there, select "edit profile" and look for a little pencil icon in the top corner of the profile picture that is shown.
It may be possible to do the same thing using a mobile browser by requesting the desktop site version of the page, but I have not actually done that. I do not think it is possible using the eBay app.
