I myself canceled GST registration in July 2019, now I have to restart it. How to start it again?

There is no option to again start with the same number if already cancelled the registration . in case of restart you may require to apply for new registration.

@ agree with Raj Kumar Jain

If registration is cancelled on the request of the registered person, such cancelled registration number cannot be restored. 
Yes, you can apply afresh against same PAN and same name and same premises. Pl. ensure that no Govt. dues are pending against the old registration.

If registration cancelled request send by your side and officer granted your request then you can't restart you gst registration.
or if you do to file your gst return from last year and officer cancelled your registration then you restart this registration by filling all return then you can apply revocation request..

If the cancellation is accepted by officer & the order for the same being Issued , the there is no wayout to revoke .

You need to apply for fresh Registration.