To type where you want to, you need to move the insertion point.
There are different ways to move it.
With your mouse you can move the pointer just to the left of where you want to insert text and then click to insert the text. Once you start typing, the existing text moves to the right as you enter the new sentence.
Press the UP ARROW key on your keyboard to move the insertion point up one line at a time.
Then press the LEFT ARROW key to move the insertion point left, one character at a time. Or press CTRL+LEFT ARROW to move left one word at a time.