Remember, in our criminal justice system, you are innocent until proven guilty.  Therefore, in criminal cases, the burden is not on the defendant to "prove innocence."  Rather, the burden rests with the state to prove, beyond a reasonable doubt, that the defendant is guilty.  With that in mind, it appears there may be exculpatory evidence in your case which would negate a finding of guilt beyond a reasonable doubt.  The bank should have a record/image of the allegedly altered check.  Moreover, the bank should also have a record of the amount of money that they cashed the check for.  If your version of events is true, these records could potentially be used to exculpate you (i.e., negate guilt).  I recommend that you discuss this potentially exculpatory evidence with whomever you retain to represent you, whether it is a private criminal defense attorney or a public defender appointed by the state.