Note:  Please set the GSTIN for all customers/suppliers by going to Master->List Customers

To download the GSTR1 in excel format.

Goto Reports->GSTR1

In the following screen GSTR1 Summary screen, click on the 'Download GSTR1 Excel'

GSTR1 excel file will be downloaded to the browser download folder.   Normally, it takes about 30 secs to prepare and download the file.