How to get string in following format in SQL

To be honest, you're probably storing it wrong if you need to do that.
Storing data as NVARCHAR is simple, and easy for your app code, but it's a PITA to work with when you want to do anything with it, as you are string to see.
Instead, I'd store it as two non-string columns, a "start" and an "end" time column. The column content would depend on what I wanted to do with the data, and what it's for, but the simplest solution is to store it as an integer: "hours since midnight". Then your formatting it for your string becomes pretty simple: I'd write an SQL function that accepts an integer, and returns the time string:

CREATE FUNCTION [dbo].[HourToString] 
(
	-- Add the parameters for the function here
	@Hour INT
)
RETURNS NVARCHAR(4)
AS
BEGIN
    RETURN CASE WHEN @Hour = 0 THEN '12AM'
	       WHEN @Hour < 12 THEN CONVERT(NVARCHAR(4), @Hour) + 'AM'
		   WHEN @Hour = 12 THEN '12PM'
	       ELSE CONVERT(NVARCHAR(4), @Hour - 12) + 'PM'
		   END 
END

And everything else gets easier to work with later as well. Such as checking if you are in that time period, and so on!