Yes you can.
The easiest way is to call CS and they will talk you through the changes you can make and confirm if you may need to pay double in the first month (it all depends on your current date and the date you want to change to).
http://www.o2.co.uk/contactus
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion.
This is not Customer Service so we are unable to help with account specific issues.Currently using:
Pixel 7a (O2 & Lyca), One Plus 6 (Sfr), iPhone 12 Pro Max (Vodafone)