How to change DateTime time format to military time format (Visual Cobol)?

"Managed COBOL" can mean COBOL for JVM or COBOL for .NET. When you ask a question about managed COBOL, please specify which variety you're talking about.

For any question, it's a good idea to include the product name, version, and platform as well. We're not psychic.

If you're talking about the .NET Framework's DateTime class (actually a structure, aka valuetype, not a class), that's a .NET Framework question, not a COBOL one. The answer is in the .NET Framework documentation:

https://docs.microsoft.com/en-us/dotnet/api/system.datetime

The Framework provides extensive support for formatting DateTime objects. You can format them directly to a string:

declare timeNow = type DateTime::Now
display timeNow::ToString("HH:mm:ss")

Note the "HH" specifier gives you the hours using a 24-hour clock and two digits.

Or you can specify a format for a DateTime object while doing composite formatting, e.g. with the String class Format method:

display string::Format("It's {0:HH:mm:ss}", type DateTime::Now)

There's a set of standard format specifiers, or you can use a custom format string as I've done here. Again, it's all in the .NET docs.