Mtn wrongfully billed me

I have been with mtn for over 5years and this is the 1st time I hear that I must pay for early upgrade but I went to the store as I was due for one... Mtn wrongfully billed I was due for an upgrade on the 30 april 2020 as per the *136*5# I went to an mtn store and upgraded my 2 contracts and the total was R588 per month so on the 20.05.2020 I was billed R1071.00... i called the call centre after 1 hour i got through to an agent she 1st advise me that i haven't paid my account from February and she put me hold for 20mins to be told I got charged for early upgrade which is totally incorrect because I was due for an upgrade...I have reversed the payment and made my monthly payment via eft of R588 I want mtn to rectify the unbilled amount because I was due for an upgrade