Exactly sub
Easy, just carefully remove a couple strands from the wick. I believe 1 strand equals about .5mm? Please someone correct me if I'm wrong. So if you remove 2 strands (don't pull the strands out, hold onto the strand and twist the wick, gently pulling the strand out) you will have 2.5mm wick.
Just double check the strand size.. I think it's .5mm though