Extension of Chilean Visa

If I got the 90 days and they are finishing the first week of April, I will be in a course by then (in Koh Phangan) so can't leave for more than 1 day and will need to extend my visa hopefully by land (cheaper).

If I already got 90 days and I'm staying till the end of my visa, what options do I have to extend it for one more month? There is an inmigration office in Koh Samui but I don't know how it works for countries that already got the 3 months visa

Thank you! :)