Hi Cananangın,
I am John, an Independent Advisor and a Microsoft user like you. I'm here to work with you on this issue.
Please refer to the link below on how you can convert a Finish milestone to Start milestone.
http://www.eastwoodharris.com/ms-project-training-tips-how-to-set-up-start-and-finish-milestones/
Note: This is a non-Microsoft website. The page appears to be providing accurate, safe information. Watch out for ads on the site that may advertise products frequently classified as a PUP (Potentially Unwanted Products). Thoroughly research any product
advertised on the site before you decide to download and install it.
I hope it worked out well for you. If not, please reply back to this forum to let me know. I will be glad to follow up and help you.
Regards,
John