When you're editing content in Microsoft Office you're in one of two basic modes: Insert or Overtype
Insert mode is what you're used to. As you type, any characters to the right of what you're typing move over to make room for your new text.
When you are editing text in overtype mode, you are typing text over your previous text. In this mode, typing new characters replaces any existing characters to the right side of the insertion point.
To switch back and forth between insert mode or overtype mode on a PC just press the Insert or INS key on your keyboard. Macs don't have an insert key, they're just in insert mode by default.