There are many such ways to do this; have a good credit record, not default on loans, have high interest rates, have a strong legal system, have a good business environment, have high quality workers, good infrastructure, political stability, high exports, strong financial system, etc.
Now many of those are long term sort of things, but short term, interest rates, balance of trade changes, political stability, etc.
These are ways to strengthen a country's currency. But i have one question in mind that if one country's currency is more than another country then also its economy will be stronger?
Actually, Yes. In economics 101, we learned that a country's currency become stronger when there is strong demand for the country's currency. Therefore driving its value up. This is caused by a high number of foreign investors investing in the country, high demand for the country's exports and a low inflation rate.
Subsequently, countries with higher currency also have higher standards of living. This is because the country's people can afford more imports with their money and therefore buy more goods made in other countries.