Basically you have 2 options when your on selfserve.publicmobile.ca/Overview/plan-and-Add-ons/ChangePlanLandingPage/?PageRedirect=RedirectToChnagePlanPage:
1)You can either "Change Plan Now" which would reset all your usage including your billing cycle.
2)Second option is to "Change on next renewal date", as it says, this would change your plan to the one desired in the end of your cycle and it would charge you accordingly as well.