Turning security deposits already held in to last months rent

[Scott Esmail]

I own several rental properties and tenants have given me 1st months and security deposit when they have moved in, I now am holding a lot of security deposits that have amounted to a good amount of casht, I understand that security deposits can not be touched until they move out, is there a way that I can send tenants a letter or something telling them that we are going to use it for there lasts month rent instead  and then I should if I am correct be able to use the funds ?