Good question. Assuming you are talking about an active / already built MS Project schedule, there are a few things you will need to do:
1. To change Start and Finish to show Hours: click on >Tools, >Options, >View, click on the dropdown for Date Format and change to 12:33PM and click on OK.
2. To change Duration to show hours: Click on >Tools, >Options, >Schedule and change the option “Duration is entered in” to Hours
This article will show screenshots for steps 1 and 2: