A Chandala is a person who disposes corpses. As Hindus don't even touch a person if anyone in his household is dead, a chandala cannot be touched by anyone as he always deals with disposal of corpses. After disposing, is there any ritual for him to purify himself so that anyone can touch him?
There might be various methods described in Puranas, Itihasas and Smritis regarding rituals or methods for purification of Chandala.
However here I prefer to quote a method given in the Vedas. Srikantha Shivacharya in his Brahma Sutra Bhasya (Srikantha Bhasya) quotes this quote which I discuss in my question here:
The Sruti says:
"If even a Chandala, if he should utter the word 'Shiva', one may talk with him, dwell with him, eat with him."
Now as discussed in answer given there the above quote is found to be from Katha Brahmana of Krishna YajurVeda.
Basically what the quote is saying is if a Chandala pronounces the name 'Shiva' it makes him so pure that one can even eat and dwell with him.
By firm knowledge of the self also, a ChAndAla can purify himself. From the inference of Manisha Panchakam by Adi ShankarAchArya. The position of Guru is full of purity; calling someone guru implies the purity of the callee.
If a person has attained the firm knowledge that he is not an object of perception, but is that pure consciousness which shines clearly in the states of waking, dream and deep sleep, and which, as the witness of the whole universe, dwells in all bodies from that of the Creator Brahma to that of the ant, then he is my Guru, irrespective of whether he is a chAndAla or a Brahmana. This is my conviction. Verse-1.