Invoice uploaded as B2C instead of B2B in GSTR1 for f.y. 2020-21.

Under GSTR -1 
you will see a option under Table 10 Amended B2C 

Amend Details you have to reduce the amount of the invoice and taxable value which need to correctly show as B2B 

Now Add B2b inovice details under Table 4 along with Amount, date and GST tax details,etc.

Make sure you check the final figure and file the amended in the return.