from where you want to change the time format. please specify clearly if you want to change on your computer, follow this instruction
Microsoft Outlook uses the country/region settings for your computer to determine how the times appear.
For example, in Calendar in the month view, if you are using the default settings for the United States, Outlook displays a 12-hour clock. After 12:59 P.M., the time is displayed as 1 P.M. Other countries/regions use a 24-hour clock.
You can change how the clock appears in your display. However, changing this display is an option provided by the operating system, not Outlook.