How to change from itemized deduction to standard deduction in 2015

Why would you need to do that?

2015 is no longer supported.

Also you cannot claim any refund for any tax year prior to 2018 since the 3 year statute of limitations has run out.

Are you amending a 2015 tax return for a tax refund?  If so, it is to late to receive the refund for tax year 2015.  The tax return with a refund had to be filed or before April 15, 2019.