Can my landlord increase my rent mid-lease?

I live in Missouri and I'm in the last couple of months of a 12 month lease. I recently received a letter informing me that my rent will be increased next month due to "expenses with my rental properties that are prompting the need for a rental increase." The letter is dated 30 days from the effective date of the increase, but I received the letter in the mail a week later. There is no mention of increasing rent in the lease. Just want to make sure this is legal and being done properly. Thank you.