iPhone; There are several ways to quickly edit text on an iPhone. But the best way is literally under your fingers.
During its ten-year history, the Apple iPhone has suffered too many improvements. Some of them involve typing, but word processing, on the other hand, is still a big one problem, as the small screen does not allow accurate and quick positioning of the cursor where you need it.
The good news: iPhones with 3D Touch (6S, 6S Plus, 7 and 7 Plus) have a hidden mode which makes cursor movement easy, fast and accurate.
The default iOS keyboard on any iPhone 3D Touch has a hidden trackpad.
To activate it, press and hold any one key until you see the keys are empty. You will feel a tactile tap (h Apple Lossless Audio CODEC (ALAC), it calls Taptic feedback).
Your keyboard is basically a trackpad and as you slide your finger around the keyboard, the cursor follows the screen.
If you want to select text, relax it pressure that you hold on the screen to go where you want, then press firmly again to start highlighting the text you're interested in.
You can try the above trick when editing a document.
What about older iPhone?
Unfortunately there is no mouse. But, no matter what iPhone you have, they are there two ways to move the cursor to edit some text. The first way is to tap the screen where you want the cursor to go. This works, but it's not terribly accurate either.
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.