If your terminal is using readline/emacs bindings, Meta + B will go backward one word. Ctrl + a will move the cursor to the beginning of the line. There are of course equivalents if your terminal is configured to use the vi/vim keybinds.

On macOS, you can hold Option and click to emulate the correct amount of left or right arrow presses to move the cursor.

So, no, you don't need to type the whole command again. I would personally probably hit Ctrl + a and move my cursor forward a few words to where I need to add the flag.