Need to reverse times

Hi currently my plants have been in 3 weeks flower, the lights are on from 10pm to 10am however I want them to be on from 10am to 10pm instead.

Whats the best way to change it over, do it slowly an hour a day or just leave them in darkness for an extra 12 hours 1 day?

No, do it it one shot. Extend your current dark period by 12 hours, giving your plants one 24hr dark period. Then just change the settings on your timer to reflect the new schedule.