I have a question about Wudu. What are we supposed to do about that part in the ears which we cannot completely reach with our fingers, because the fingers are too big and the hole is small?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Imam Abu Dawud and Ahmad reported a Hadith narrated by Miqdam Ibn Ma'adi Karb (Radiya Allahu Anhum) that the Prophet (Sallallahu Alaihi wa Sallam) wiped over his head and over the outward and inward sides of his ears.  The method of wiping is as follows:

To put the index fingers into the ears and wipe around the inner part, and simultaneously run the thumbs around the outside part of the ears.  This is enough.  You do not need to put water inside the ear, as wiping is sufficient.

Allah knows best.