Question:
1- How to select switching frequency of switcher ??
2- Which is better lower switching frequency or higher switching frequency and Why ??

 

Higher switching frequency = more semiconductor losses, faster response time,
Low switching frequency = larger inductor, more inductor losses, slower response time,

 

In addition to the stated relationships there are also the questions of size and cost and actual considerations of heating, as well. For some power levels and voltage ratios a higher frequency makes more sense, because the overall efficiency can be greater.
But also consider that the opinions of those with a lot more switcher design experience agree completely with my opinion, which is that the design of an efficient practical switch-mode power supply is the most complex and difficult electronic design effort that there is, currently.
So really the correct answer would be: "It Depends."

 

It would be a good idea to simulate the Circuit in a good Circuit-Simulator-Program.
Once You get the desired results,
then double the size of the Output-Capacitor in the real Circuit to better insure low-ripple on the Output.

Many Data-Sheets will also recommend a second Filtering-Stage for reducing Ripple.

It's also a very good idea to follow the manufacturers recommended
basic Circuit-Schematics if they will fit your needs.

If You would state what You are trying to accomplish in as much detail as possible,
You would probably get much more useful recommendations than simply "high" and "low".
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Question:
1- How to select switching frequency of switcher ??
2- Which is better lower switching frequency or higher switching frequency and Why ??

There are no simple answers to your question. The component selections are not performed independently but as part of a larger picture. To answer your own question try doing the same design at three different frequencies. Simulate them and compare the results. Then build them and ask yourself: "do the real designs match the simulations and do the trend in the same direction?" Then you will have some empirical data to support a provisional conclusion.