The Insert key

On most computer keyboards is a button called Insert (INS).
This key is usually above the arrow keys, and beside the Delete key

Its main use is the switching between two modes of entry of the text insertion and replacement of the text.

Insert mode

By default, Windows uses the insert mode for entering text. Here's how it is:

Insert mode, typed characters are inserted at the cursor location.

Example (the symbol | point out the cursor):

a cha | nge

By typing the letters' L ', ' L ' and' E 'it will give:

a challe | nge

Replacement Mode

But there is also another way: Replacement Mode
In replacement mode, writing each new character replaces an existing character, the one right next to the cursor.

Example (the symbol | means the cursor):

a cha | nge

By typing the letter 'R' it will:

a a char | ge

• • Problem: The word I just type replaces the one I previously typed

The problem is simple: you had unwillingly press the INS key
So you have changed the modes (Insert> replacement).

To return to the proper mode, press the button again and make a test.