how do I reset my width and height?

How do I reset width and Height of photos already added to site. there is a button that says reset, in image settings but when I push it. gives me no option to change numbers or to manipulate the rather large image it created when I hit the rest in the first place. Thank goodness for the undo button, really...
Limited on screen time as my medication side effects make me sick to look at screen so any short cuts would be lovely. If this a matter of an upgrade experience, no money...no funny.
Reason for the sizing is. ...images are recieved as 8x10 or larger web sized images. 1 to 1.5 mb each of storage, but sized wrong. Thought I would take in and use your experience, or try to learn for hubbys sake. hard for me tho. May have to fall back to my photoshop skilzz.
Anyhoo, Images on page are icons to right click to see larger pics... the pics are not turning out larger.