I changed my mind.
'Cambiato' — cam-BIA-to. Three syllables.
A simple, honest explanation for a return when there is nothing wrong with the item. Use in relaxed, informal shops. In more formal boutiques, a more diplomatic phrase is preferable.
'Ho cambiato' is passato prossimo of 'cambiare' (to change). 'Idea' (idea/mind) is feminine. The phrase is a direct calque of the English idiom. In Italian, 'cambiare idea' is perfectly natural for changing one's opinion or decision.
Ci ho ripensato.
I thought it over again.
More reflective tone — implies you reconsidered.
Non è quello che pensavo.
It is not what I thought it was.
Implies the item did not meet expectations.
Non mi convince.
It doesn't convince me.
Idiomatic — 'it doesn't win me over'.
A 'nota credito' (credit note) is very common in Italian retail — it is store credit, not cash. Many shops default to this. If you want actual money back, specify 'rimborso in contanti' or 'sulla carta'.