Lease Termination / Renewal options

[Sam Banerjee]

Hi: 

I bought a rental in NJ Nov'2015 - it was already occupied.  The tenants are good and had no problems signing a new lease from Nov till Jun 30th. 

Now they want to do month to month - which for me is not feasible. I am unable to reach them - they are not picking up phones / nor answering the door.

I would like to give them 2 options - 

1. Renew for 1 year (same terms as existing lease -- I got a standard residential lease from staples)
2. Vacate the place

Is there any standard / legal cover letter that I can send via postal mail ? 

Is there a minimum notice period that I will need to give them ? (nothing is mentioned in the lease) 

I'm a rookie at this still :) - so any help will be much appreciated.

Thanks

Sam