Income Tax Prepared WRONG!?

I had my taxes done yesterday for free (I am currently unemployed) at an account aid place-here is the problem...my Uncle always did them (for 19 yrs.) for me through H & R Block, always had a nice refund, always got me all the $$ possible. Last year I only worked 1/2 the year, plus a small 401K was closed out (by a former employer) so I have a 1099 as well, which they also took out $500 for federal taxes. My situation has not completely changed over the years (I have 3 kids, but this year my ex's have claimed them,) now the result is--I have to PAY! I am comparing my taxes done in previous years by my Uncle and I do not believe this place did my taxes correctly! I am going to run it thru turbo tax later today, but can I just make the changes on the doents I was giving yesterday and mail back into the IRS? Will I get in trouble for changing the numbers (even if I FIND they are correct) b/c this acc't aid society has their name ont he docs as the preparer?? I have NEVER had to pay in the past (even when I made really good $ as a legal t.) and last year I ONLY made (with the 401K) $25,105.00??? I need help or a direction as to what to do--if I do owe and don't contact the IRS today or get it postmarked I know I can be in trouble-but for example-he didn't even check the box for exemptions that said "children you have but don't live with you do to divorce" I should be able to put at least ONE of my kids in there. (2 are minors-one is in college but was with gramma 6 months of last year AND she claimed him before I COULD!) Am I screwed or can I amend this docs? thanks!