Is this a paid ticket?
If yes, then change after you have flown the first leg. Then applicable change fees according to your fare rules will apply. Stopover in Singapore always possible with additional airport fees only.
If you change (even its only the last sector) before you have flown the first segment, this is considered a cancellation and your entire ticket must be refunded and repriced according to current rules, fees etc. So its not the best thing to do.