If your client has an amount eligible to defer, and client does not want to defer or wants to defer less than the max, open Sch 3, go to Pt 2, line 12e.  Then click the quick zoom on that block.  this will take you to yet another 1040 worksheet.  Just above where the cursor lands you will see a place to change the amount to defer (a box colored brownish yellow).  If the client cannot defer then there is nothing to do and if you put in an amount PS will tell you the amount is too much.  40 years of this and I hate it more every year but it is what it is, as they say.