To configure/change the GST number, follow the below steps:
1. Log on to the Sandbox Dashboard and click on Profile Icon > Business Details
2. In the Business Details settings, you will see a checkbox to enable the option for "I want GST Invoice" and then enter the relevant details like GSTIN and Place of Supply and click on "Save".
You can update your GST Number anytime.
3. Once you save the changes, going forward all the subscription & wallet invoices will contain these details.
In case you need any further help, feel free to raise a ticket here