You will need to file a 3115 to make your correction.

https://www.irs.gov/forms-pubs/about-form-3115

File this form to request a change in either:

• an overall method of accounting or
• the accounting treatment of any item.

 You are requesting to change the accounting treatment of your rental property.

Your post sounds like you may be doing this for yourself.

Whether or not it is for your personal return, I would not try to complete the 3115 myself.  I would find a tax professional experienced in this form to complete it for you and help you through the process, It is complex and you don't want to screw it up.