An insertion point marks where new text goes. A selection marks text that an edit will affect. In this paper editor, square brackets show a selection; they are not part of the stored sentence. Check exactly which letters and spaces are selected before typing.
Worked example
In The [red] kite., type blue to replace only red. The result is The blue kite. The spaces and period were outside the selection, so they stay.
The Paper Editor
Start with The red kite. Select only the three letters red, shown as The [red] kite. Type blue once. The selected letters are replaced together, while the rest of the sentence stays unchanged. Next place the insertion point at the very end, after the period, without selecting anything. This is a fictional text model, not an instruction to change real schoolwork.