To add your GST number to your order invoice please follow the following steps
Step 1: Go to Settings
Step 2: Click on Company Profile
Step 3: Go to KYC Details and enter your GST Number, Pan Card Number and upload GST Certificate, PAN Card and Signed agreement and click on Save
After saving this you will get a message on the top right corner stating "Company Details Saved Successfully"
This means you have added your GST number to your order invoice successfully.