How to active the cancellation GST

1 - login on GST portal

2 - Services > Registration > Application for Revocation of Cancelled Registration

Fill the details required 

 

 

If you yourself cancelled your GST registration, than you can not activate it again...as it's cancelled by yourself only.

if Your registration cancelled by GST officer on non filing of return, than you can make application for revocation..
Make sure you filed all the pending Returns before applying for revocation application.