how to dispute charge from itunes

See the Receipt you should have received for the charge, or log in to https://reportaproblem.apple.com

See here for more details: If you see 'itunes.com/bill' or an unfamiliar charge on your statement - Apple Support

I believe you want to dispute because of some unfamiliar charge in which case follow the instructions in this post:

https://support.apple.com/en-us/HT201382