According to Quran if you have skipped fasting in the month of ramadan, because of some reason then you should make up those fasts by fasting on other days i.e, in months other than ramadan. When you can with the intention of fulfilling the fasts of ramadan. This can be done any-time in a year.
Source - Quran Al-Baqarah, 2:184
[Fasting for] a limited number of days. So whoever among you is ill or on a journey [during them] - then an equal number of days [are to be made up]. And upon those who are able [to fast, but with hardship] - a ransom [as substitute] of feeding a poor person [each day]. And whoever volunteers excess - it is better for him. But to fast is best for you, if you only knew.