The easiest way to replace a character or short string is to write your changes on top of the characters to be replaced.
To change a single charcter:
Write the new character directly on top of the current character and tap Enter.
To change a string of characters:
Select the set of characters you want to change by circling them.
Write your change directly on top of the selected characters and tap Enter.
Your new entry can be shorter than the current selection. For example, if you start with the expression "a + b + c" and select "b + c", you can write d anywhere on the selected characters and tap Enter to get "a + d".
If your changes are longer than the original text:
Select and delete the text that you want to replace.
Use the insert space gesture to create enough space for your addition. The insert space gesture is described in the Adding to an expression topic.
Write your changes in the inserted blank space and tap Enter to add them to the expression.