Correct spelling, explanation: conversion is a noun derived from the verb to convert with the use of the suffix -ion. In the case of words ending in t, the process is not so simple, as the t usually has to be changed to s or ss. This is mostly caused by the efficiency and pronunciation, however, based on that, conversion is the only correct form.

Definition of conversion:
Noun
1. An act of turning a thing into another
The idea of conversion of the old factory into a library was definitely a hit.
2. Changing one’s religious beliefs from one system to another
When I moved to India I went through the process of conversion from atheist to Buddhist.

Incorrect spelling, explanation: despite the fact that it may seem logical that when a noun-creating suffix -ion is added to the verb not much changes, convertion is not a correct word. Just like emit → emission or submit → submission, convert → conversion. When in pronunciation the t changes to s, the spelling is also changed.