LX22

Hi everyone! Sorry my question my seem a little silly but as I'm not very comfortable with using my miles, I thought I'd ask here for advice.

I'll be flying from MEL to NRT through SIN on SQ during June, and I just saw on Aegean's upgrade award chart that both SIN and NRT are in their "Far east" zone and that upgrading from Y to C would cost me 15'000 miles, I was wondering what to concretly do to book this upgrade, and if there were any surcharges that were to be added? Do I have to call them while in MEL to request the upgrade? I won't have the 15'000 miles in my account before about 1 week before the actual flight. (Edit : My booking class is H if that helps)

Thank you!