换RMB = 把外币兑换成人民币 = to exchange foreign currency for RMB
- 我要换人民币 = I'd like to get some RMB.
- 我要用美元换人民币 = I'd like to get some RMB for my USD.
- 我要换钱,美元换人民币 = same as above
To lose the ambiguity, add a preposition (成/掉) like M Mira suggested.
This is also applicable to 借 where prepositions like 向/给 would leave no doubt as regards the direction of the transaction.
- 他向你借了钱? 嗯,借了点. = He borrowed money from you? Yup, (he borrowed) some.
- 他借了钱给你? 嗯,借了点. = He lent (to ) you some money? Yup, (he lent me) some.