Just depends on where and how you want to do it. I mean if you are using a Microsoft word:
Steps: Delete Nepal and Type US. Thats it.
However it gets little complicated if you are using photoshop. you probably have nepal written in a jpg file. So this is how you do it:
Steps: Use a rectangular marquee tool and select Nepal and press delete. Then select Text: and just type US.
I hope this help.
Next time avoid asking stupid questions...