How to tell tenant they have to leave

[Jared Lundy]

I am pretty new to RE investing and I am in the process of buying a little rental. There is a tenant in the rental who has lived there for 13 years but is only paying $400 month but there is no lease in place.

The owner has told me that they have never had a problem with the tenant but the owner feels the tenant is vindictive and will trash the place if the tenant finds out they have to move.

According to the owner the tenant cannot pay the $750 - $800 month rent that the house can rent for. 
I am putting it under contract today and the tenant still has no clue the house is being sold. How do I go about telling the tenant and getting them to move out of the home they’ve lived in for 13 yrs hopefully without them trashing the place?

Thanks