You change your dependency status in the My Info section of the online program. Just edit your personal information and you will come to a screen labeled Let's check for some other situations and then check the box labeled Another taxpayer can claim me as a dependent on their tax return.
Before starting to amend the tax return, wait for the tax refund to be received or the taxes due to be paid and processed by the IRS.
See this TurboTax support FAQ for amending a current year tax return - https://ttlc.intuit.com/turbotax-support/en-us/help-article/tax-return/amend-federal-tax-return-curr...
