Basically, you need to leave the existing asset entry alone. Any changes to it will affect depreciation in a negative way.
With whatever you do, if you want to confirm the current year depreciation is correct when all is said and done, see IRS publication 946 at https://www.irs.gov/pub/irs-pdf/p946.pdf Use the MACRS worksheet that starts on page 37 of that document. For line 6 of the worksheet, table A-6 on page 72 applies.
