How to Correction in GST Challan

If the challan is not paid, then you may pay under a new challan with correct values.

Where the challan has been paid, then PMT-09 could be filed for shifting balances from one head to another of cash ledgers.

Yes, If the challan is not paid, then you may pay under a new challan with correct values.
Old unpaid challan will automatically got invalid after 15 days of generation of challan.

And if the challan has already been paid, as mentioned by you.. then you can file form PMT-09 online for shifting balances from one head to another head under cash ledgers (i.e tax to fee column)

In your case as you are already paid amount, you are required to file PMT 09.