How can I reverse 6gb for week to 3gb for a month which I mistakenly press

Hi Awolana, MTN do reverse wrong data activation if you have not used the bundle, just contact their support channels, they will help you to reverse it. If you did it via your bank account, you will be required to provide the reference number of the transaction and other details.
You can contact their support via any of these channels.
1. MyMTN App
2. Facebook: @MTNLoaded
3. Instagram: @MTNNG
4. Live Chat: http://bit.ly/2OC5bHz
5. Email: customercareng@mtn.com
6 .WhatsApp: 09033000001