When you are routing you should see the via diameter and drill in one of the icon based menus (at the top on my setup, but they can be moved). You can click on it and get a list of used drill sizes (or a default list) but you can also click on the size and type another followed by enter.
It is worthwhile getting used to using the command line. You don't have to click on it - just start typing and hit enter. You don't have to type all a command - just enough to be unambiguous. So, for delete type DEL or maybe even DE. To change text type CHA TEX.
Keith
- - - Updated - - -
Just to add, there is also a command history. Hit the up arrow key and it will allow you to scroll through previous commands. It can be handy to repeat a command or recent ones.
Keith