Hello Guys

I need your advice. My folks has an apartment in the province and the tenants rent is 5k per month for a two bedroom unit (6 rooms, 2 floors, and 2 bedrooms units each). All of the tenants have been renting for 5 years already and with the whole increase of prices (groceries, gas, etc) my folks thought of increasing the rent for the tenants to 500 pesos

Are my parents allowed to increase the rent and they will inform the tenants that this will take effect by next year? I would appreciate the advice

For additional information: the tenants have a contract with my parents, each tenant has a minimum of 1 year stay, 1 month advance and deposit, my folks provides a 14 day grace period for tenants in case may delays at inform lang nila parents ko. Tenants pays rent every 15th of the month, bills are paid accordingly and mabait naman mga tenants.

Thank you