Hello Nimeng.
I don´t want to discourage but design a switched mode power supply is a very challenging task, i.e. to obtain a good design. Your intention to modify your power supply also I don´t think is a good idea. It is not just to change some components. With the information you provided is not enough to say but maybe to obtain the power you want also have to change the PCB, or design a new PSU. In the case you should redesign your supply I suggest you to buy a new one with the specs you need.
If you want to know how to design a SMPS is ok, I encourage to you to do that but firsst you must study a lot, this is not calculate resistors and capacitors. I suggest you to read the ebook Power Supply Cookbook.
---------------------
Best regards. Yosmany325.