andaba cambiando un billete
please tell me the meaning of this sentence
"I was cashing a bill."
cashing as in exchanging a $20 bill for smaller bills that equal the same amount.
First scroll down towards the bottom of this article and read about the gerundio (Spanish gerund) used with verbs of motion. Note that they express connotations to the normal verb meaning.
I don't think that this particular article mentions andar+gerundio, but I think that it means "to go around doing whatever the verb is".
I won't guess at your particular sentence because billete can have so many different meanings from bill (money) to tickets.
According to Lazarus1907 reply in this discussion: andar+gerundio=estar+gerundio which would confirm tamalmalamar's answer.