Thank you for using justanswer. During the time you were here under the F1 visa, you were, as you already stated, exempt from paying taxes on any interest you earned on your bank accounts here in the US. Now that your visa and status has changed due to your H1visa, you now must file, and pay taxes on, your world wide income, just as all US citizens and Resident Aliens must.
The only way to get your money back is to file your tax return, claim the interest earned on all of your bank accounts, report the $6800 as Federal tax withheld, and you will receive the difference between what was sent to the IRS on your behalf (the $6800) and your actual tax liability. Please see below:
Taxation of Resident Aliens
I hope this helps.
You will file Form 1040X (Rev. February 2009) Amended U.S. Individual Income Tax Return where you will report the interest earned on the account and the $6800 Federal tax withheld.
Unfortunately, I can't answer your question re: what kind of refund you will get. The amount of tax that an individual pays on his or hers income is a product of their filing status, tax bracket, type of income, eligiblility for certain credits, etc.
You might want to have a professional tax preparer actually fill out the 1040X form, but I have linked you to both the form and the instruction just in case.. Please see below:
I hope this helps