You say it's been wrong for "several years". If you've been depreciating incorrectly *or* using an incorrect method *or* not depreciating any one or more assets for 2 or more consecutive years, then amending is not an option. You have to file IRS Form 3115 for fix this. Now while the program does include the 3115, very little support in the way of "help" is available for that form. I *HIGHLY* suggest you seek the services of a tax professional in your area; especially if your state also taxes personal income. Doing things wrong "WILL" cost you dearly, and what you pay for professional help will be a pittance in comparison to the fines, penalties, late fees, interest and back taxes you will be assessed if not done correctly.
Typically, folks don't catch their mistake until the tax year they sell the property. That's when all the additional costs really kick in and can significantly reduce (if not eliminate) any gain you may realize on the sale. Please seek professional help with a tax pro in your locale with this. I do not recommend you attempt the 3115 yourself.
Check out IRS Publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf beginning at the bottom of page 13 and on to page 14. Basically it states:
Generally, you must get IRS approval to change your
method of accounting. You must generally file Form 3115,
Publication 946 (2023) Chapter 1 Overview of Depreciation 13
Application for Change in Accounting Method, to request
a change in your method of accounting for depreciation.
The following are examples of a change in method of
accounting for depreciation.
• A change from an impermissible method of determin-
ing depreciation for depreciable property if the imper-
missible method was used in two or more consecu-
tively filed tax returns.
• A change in the treatment of an asset from nondepre-
ciable to depreciable or vice versa.
• A change in the depreciation method, period of recov-
ery, or convention of a depreciable asset.
• A change from not claiming to claiming the special de-
preciation allowance if you did not make the election
to not claim any special allowance
