Why are the values of currencies different for different countries?

it is found that in each and every country in the world has its own currencies which have different rates.for example one US Dollar equals to 45 indian rupees approximately.how can one determine that? if one country ,claiming to be sovereign ,plans to revaluate its own currency and wants to make it equal with that of the US, is that possible? if one renegade state decides not to honour intrnational currency obligations, what is going to happen?