You need to use the form 1040X - http://www.irs.gov/pub/irs-pdf/f1040x.pdf - there is no other way.
In the header -
(B) - mark for which year you are amending;
(C) - Explanation of changes - you will write - "Changed from itemized deduction to standard deduction"
On the line 2 - you will put your standard deduction amount.
Follow instructions - and you should calculate your additional refund.
Let me know if you need any help.
I have already completed a1040X and it comes out on line 19 that I owe $509 which is $250 less than than I paid last year by check which was $759. Do I send this form in with
a copy ofthe 2008 1040 I filed last year or is there another form I fill out?
You do not need to send a copy of your 2008 tax return - the IRS should have that copy.
On the line 15 - you should have $759 that you paid last year,
and line 20 should be $250
line 19 should be empty.
If you previously sent W2 with your original tax return - there is no need to sent them again.
You should only send forms which are changed - most likely - you do not have to send any additional forms.
Do not forget to sign the form 1040X.