Hi, you can apply online. I believe you can renew but they don't change the time frame in your original Police certificate ( which means everything else remains the same except the date of issue) if you haven't stayed in SL during the period then you can go with this option.
If not you can apply for the new one online with new time frame but if you are from Jaffna, then you need to get the documents signed again by your GS and redo all other procedure.
If you are from Colombo, I think requesting new one through online would be straight forward (just that police will visit your place where you lived) .
This what i know. I also tried to renew mine about an year ago but without change in timeframe, Canadian embassy won't accept as they want it till date. Since they change the policies often, it's always better to call police head quarters in Colombo and get it clarified. They are very helpful and will tell you exactly what need to be done.
Application for Clearance Certificate (police.lk)
Telephone :- 011-2439185 / 011-2421111