For the biggest savings you need to upgrade to a sim only tariff.
Choose the deal you want
This will automatically redirect you to the O2 website
Just follow the prompts and choose the UPGRADE option
Complete the deal and nothing on your account changes except the tariff at the new cheaper cost
Payment date is the same and your next bill will be cat a cheaper rate and will be made up of the pro rata cost of your old tariff and your new tariff
The change should show in My O2 within 24 hours
Volt benefits will still apply as will possibly a 3 month extra such as Disney+
You can also go in store to see what they can offer or contact sales message them on Social Media if that works best for you -on Facebook (https://o2uk.co/O2CFB) , Twitter (https://o2uk.co/O2CTW) or Instagram (https://o2uk.co/O2CIG)
Or you can call them on 0800 081 0255 but neither will be as cheap as USwitch.