What are you seeing that you think you can't? Is it that when you type in the new number that it just doesn't go in? If so then try doing the function on a non-mobile browser like a laptop/desktop.
Or is it as the other user said, you can't seem to get there to do it?
@Isack - ensure you reactive or resume your services if you have not already done so. So, if you listed it lost/stolen, go back and resume/reactivate.
See "How To Change Your SIM Card" at the bottom of this Help Article: https://www.publicmobile.ca/en/on/get-help/articles/sim-swap-fraud
If issues with your My Account, click here to request a SIM Swap with the Public Mobile Customer Support Agents (CSA): Submit ticket for a SIM SWAP