Exchanging currencies is a form of ṣarf governed by the hadith of ʿUbadah ibn al-Samit, in which the Prophet (ﷺ) named gold, silver, and other ribawi items and said: "...like for like, equal for equal, hand to hand; but if these types differ, then sell as you wish, so long as it is hand to hand." This means that when exchanging two different currencies (the modern counterparts of gold and silver) the rate may differ, but the exchange must be completed on the spot, without deferment on either side. If the same currency is exchanged for itself, it must be equal amount for equal amount, otherwise it is riba al-faḍl. Delaying delivery of either counter-value introduces riba al-nasiʾah and invalidates the exchange.
Informational, not a personal fatwa. Consult a qualified scholar for rulings on your situation.