Can i alter a section 21 notice that's been served?

I have served a section 21 notice to gain posession of my property. I served with form 6A BUT the timing was just after the 6 month notice came in and I only gave 3 months notice as the information availalbe on GOV website was not accurate. My question is do I need to start the proceedings from the beginning or can I alter the existing notice period to 6 months? The tenant has only after 2 months of receiving the notice contacted me today.
Thanks in advance

Sorry, but you have to start again

I was afraid of this. I wish that the tenants had an obligation to reply to notices served in the same way as we do in serving them.
Thank you for quick response