If you're using BoundField, DynamicField or similar, just set
DataFormatString
property to "hh:mm tt".
If you're using TemplateField and Eval() or Bind() you can pass format string as second parameter to them:
Eval("Date", "hh:mm tt")
Of course, this will work only if you return DateTime from database.
It won't work if you return it as string.