Stopping Microsoft Word from typing over your letters depends on how they're replaced. If typed text is automatically replaced, such as capitalizing the first letter of a sentence or changing "teh" to "the," then this is Word's AutoCorrect feature trying to fix common problems. If letters to the right of your cursor are overwritten as you type, then you have "overtype" enabled. Both of these situations are correctable within Word's Options.
Click "File" from Word's top menu bar, and click "Options" from the left column.
Click "Proofing" from the left pane of the Word Options window.
Click the "AutoCorrect Options" button from the AutoCorrect Options section.
Uncheck "Replace text as you type" to disable text replacement. You may also uncheck the capitalization options, if you prefer not to enable these changes. To stop a specific change, such as "teh" to "the," click the option in the list and select "Delete."
Click "Advanced" from the left pane of the Word Options window.
Uncheck "Use Overtype Mode" from the Editing Options section.
Uncheck "Use the insert key to control overtype mode" if you want to prevent entering this mode when you press the "Insert" key.
Click "OK" to accept your changes and close the Word Options window.