The GSTN has added 6% tax rate in the item details section of all the tables of form GSTR-1, except HSN table 12. In case, outward supplies attracts 6% tax rate, it is required to upload the details against 6% tax rate in the item details section.
In respect to HSN table 12 of form GSTR-1, 6% tax rate shall be added shortly. Meanwhile, report the HSN details of supplies attracting 6% tax rate under tax rate 5% by updating the values/tax amounts as per the actual supplies made.
Source: GSTN
https://www.gst.gov.in/newsandupdates/read/543