First your upside down home needs to be short sold.
Next, you need to complete a Credit Re-Verification program:
which legally and permanently gets the short sale and late mortgage payment information
on your credit report removed or shown as paid in full,
shortly after your short sale is completed.
You basically become a "New" home buyer again.
This process can take anywhere from 90 to 120 days;
which can be done while you are short selling your home;
and requires an investment on your part.
Next you will be ready to complete your loan application,
get pre-approved, and then buy your next home.