2nd Amendment in GSTR1 on export invoice

1. Initially, we had incorrectly filed GSTR1 for an export invoice where we showed that 18% tax was applicable. Whereas, the no tax was applicable as the invoice was against LUT  

2. In the first amendment, we changed the integrated tax to 0.00 but still kept the 'GST Payment'  option as 'with payment of tax', we should have changed this to 'without payment of tax'

3. The system is not allowing us to make second amendment.

Please advise how can we make second amendment

Thanks,

Vimal Gupta