There are two unique minhagim (customary actions) applicable to a kohen for the special day of Yom Kippur, this article is a brief presentation of these minhagim;
Washing until the wrist
Generaly, netillat yadayim (negel vasser) performed on the morning of Yom Kipper is done by pouring water only until the knuckles. A kohen however, is supposed to was until his entire hand as usual ;
Since Kohanim who go up to bless are required to was their entire hand, if in the morning they had only washed until their knuckles and said the blessing על נטילת ידים – they would still be required to make another blessing.
Thus, It is better that they wash their entire hand in the morning and then not be required to make another blessing when washing prior to ברכת כהנים
-Free translation of Kitzur Shulchan Aruch 133,4
See also footnotes to Shulchan Aruch HaRav Orach Chayim ch. 613 (translated edition) regarding this as a Chabad custom.
Mikvah on the morning of Yom Kippur
Note: Consult with your Rabbi regarding immersing on Yom Kippur morning if you have experienced an emission.
Generally, even a person who has had an emission (ח”ו) on the night of Yom Kippur is prohibited from immersing on the morning of Yom Kippur, even if he is accustomed to doing so all year round (Kitzur Shulchan Aruch 133,6).
This can be contrasted with what Sefer Aguda, who quotes the Gemara that all בעלי קרי are to immerse on Yom Kippur morning, and writes ;
However, since the masses have accustomed themselves to the ruling of רבי יהודא בן בתירא who stated that immersion is not required for prayer, they are thus not permitted to immerse Yom Kippur morning even if an emission occurred. However, a kohen who had an emission – either by night or by day – is to immerse in the mikvah since kohanim generally are accustomed to immersing for ברכת כהנים.
Yet, Sefer Aguda to מסכת יום טוב quotes Rabbenu Tam who permits immersion for all בעלי קרי on Yom Kippur (see last p. of masechet Yoma).