iPhone? There are several ways to quickly edit text on an iPhone. But the best way is literally at your fingertips.
Over its decade-long history, Apple's iPhone has undergone a ton of improvements. Some of them concern typing, but text editing, on the other hand, is still a big problem, as the small screen doesn't allow for precise and quick placement of the cursor where you need it. 
The good news: iPhones with 3D Touch (6S, 6S Plus, 7, and 7 Plus) have a hidden feature that makes cursor movement easy, fast, and precise.
The default iOS keyboard on any 3D Touch iPhone features a hidden trackpad.
To activate it, press and hold any key until you see the keys go blank. You'll feel a tactile tap ( Apple calls it Taptic feedback).
Your keyboard essentially becomes a trackpad, and as you drag your finger around the keyboard, the cursor follows on the screen.
If you want to select text, ease off the pressure you are applying to the screen without releasing it to go to the point you want, and then press firmly again to start highlighting the text you are interested in.
You can try the above trick while editing a document.
What about older iPhones?
Unfortunately, there's no mouse. But no matter what iPhone you have, there are two ways to move the cursor to edit text. The first way is to tap the screen where you want the cursor to go. This works, but it's not terribly accurate.
The second way is to press and hold the screen to see the magnifying glass. This works better, but sometimes your finger can hide the cursor.
