Tenant refusing to sign a new 1 year lease

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We purchased a 2 bedroom 2 bath mobile home and placed it on a section of our property and rented it out last year. The original lease expired this month and we put together a new lease with 2 changes in it. We have had a problem with the renter paying the rent 5 to 7 days late every month so we tacked on a $35.00 late charge after 3 days late and changed the due date to the date they said that they would be more comfortable with. Gave them the new lease and got a slightly nasty reply letter stating that they have never had to re-sign a new lease on any property they have rented and that they do not wish to sign a new one and will think about it for a couple of days and let us know if they are going to sign it at all.

How would you handle this? Any ideas??