GST Invoice amt wrongly entered

GSTR-3B you cannot change, ensure the tax amount is entered correctly in GSTR-3B.

In GSTR-1, you can change through amendment table i.e.Table 9.

In the subsequent month GSTR-1 in table 9 select respective invoice, the details would be autopopulated. You can change the taxable value.

GSTIN of the customer also you can change by following the same process explained above in Table 9 of Form GSTR-1. 

not possible to change