I have had my current tenants for a year or so. Can I change the lease now and raise their rent?

The answer is "it depends." It depends on what the lease says. Unilateral raising of rent would be a very unusual provision in a standard residential lease (which Im not sure this is) and regardless probably difficult to enforce. If the term has expired you can have the tenant sign a new lease for a new price. Regardless of what the lease says, if you and the tenant both agree to a new rental amount, you can both amend the lease in writing reflecting the new price. I would recommend finding a lawyer here on to assist you with the lease review and amendment. Best of luck to you!