If you haven't prepared cash in advance, it's easy to exchange any currency into Japanese yen once you arrive in the country. All major airports have exchange points in their respective arrivals halls. Even though it is possible to buy train tickets, take taxis, and pay for most other services on offer at the airport with international credit cards, carrying some cash is recommended. You will need some during your stay.
You can also exchange foreign currency at major banks. Branches generally have convenient locations—often in front of train stations—however, be aware that many close at 3 p.m. You can research more banks’ information: