Essentially you are always in insert mode on a Mac. Microsoft Word used to have some kludgy keyboard command to trigger destructive overtype. Because it is destructive, and an easy way to ruin your work, the Mac never had it and Microsoft got rid of it. In the Mac version of Word there is a little OVR button in the staus bar (on the bottom of the screen). If you click it, it turns green and this "feature" will work just as before. I believe Microsoft plans to get rid of overtype in all future versions of Word.
The Mac way is to highlight the text you want to remove, and then type the replacement text. There are quick ways of selecting words, lines, paragraphs and arbitrary chucks of text which are worth learning.
You might want to check out David Pogue's "Switching to the Mac: the Missing Manual". It explains many of the differences that switchers find confusing. There are also numberous online guides. Usually it takes about two weeks to adjust. After a few months, most switchers come to prefer the Mac way.
Best wishes.