On Yom Kippur, bathing and washing are prohibited. However, the Kohanim who must wash their hands to give the Birkat Kohanim (Priestly Blessing) may do so, since this is a Halachically mandated washing, and not for pleasure. They may wash up until the wrist. This is the opinion of Hacham Bension and most other Poskim, including Rav Haim Palachi (Turkey, 1788-1869) in his Moed L’Kol Hai (18:13). However, Hacham Bension rules that although they normally wash their hands three times, on Yom Kippur they should only wash each hand once, as the extra washing is considered a secondary addition. In a place where the Levi’im wash the Kohanim’s hands, and they have the custom to first wash their own hands, Hacham Bension held that if that is their practice all year long, they may also do so on Yom Kippur.
On Yom Kippur, Kohanim and Levi’im may wash their hands once up to the wrist in preparation for Birkat Kohanim.