To do this you must know the password. If you don't, then you are outa luck.
1. Open a Word document in Microsoft Word, doesn't matter what document, and enter your password as usual.
2. Now click "File" from the menu (top left) and then click "Info."
3. Here you want to click "Permissions" from the right side pane and then click "Encrypt with Password."
4. Next highlight the password shown (it will probably be all asterisks/***). Once all are hightlighted press "Delete" or "Backspace" on your keyboard.
5. When prompted click "OK" to remove the password.
6. Now just click "Save" from the left side pane.
You should no longer have to keep entering your password.