considering of canceling application

hi Guys
any advise please as i applied for my father in law to come to visit me and i am sponsoring him , i applied but i realize that i made a mistake on application .i did put the date for travelling 15th April 2019 , then when i checked on GOV.CO.UK shows that have to be at least 3 months .
i already paid the fee and booked appointment but thinking to cancel . is there any link or how to cancel and get refund back .

thanks

The HO website does not state you can only put a date of travel for 3 months ahead.

It says that you COULD apply for a visa to be post dated UP TO 3 months in advance, ie apply now for a visa dated in June.

thanks ,
is that will affect the application as I put travelling date 15 of April .

No!!

You are misreading or misunderstanding what the HO website says about 3 months.