Since you are here at Stackexchange, I'd suspect you may have experience with php, html and css. If so, you could edit in the editor, or you might remotely manage the theme files via ftp with your text editor of preference. I use dreamweaver more out of habit than anything.
That said, some themes (not sure what you are using) enable you to edit the theme while logged in through the dashboard. Some let you make slight changes or adjustments to things like the header, while others let you design and customize almost everything (ex Headway themes).
My point is that the answer to your question will partly depend on your experience level, experience or strengths with the tools and code you know or use, and also may depend on the theme environment that you are using. A php novice can do amazing things in Headway themes without ever having to type any php for example. (Not recommending Headway, its just the easiest example. Thesis or StudioPress themes give you a lesser degree of theme customization as well).