When editing text on an iPhone it can be difficult to tap in just the right place to change an existing block of text. For example, to delete a character in a word a few words before what you are currently typing, or to insert a new word near the start od=f a sentence.
An "off by one" error is common, because your finger tends to be bigger than the spaces between letters you are typing. What you need is a cursor which can be moved around like you can using a trackpad n a laptop. Well, that's exactly what you have!
While typing some text, to go back to make a correction. Hold your finger down on the space bar. The keyboard should go blank. Move the vertical bar by moving your finger around on the screen. Release your finger when the cursor is in the right place. To delete a letter, move to the right of it and press delete. To insert, move to the space between words and type the new letters. To continue where you started, move the cursor to the end of the text. You can move up and down lines too, to a limited extent. To highlight a word, move the cursor into it and pause.