It’s best to inform each credit bureau that shows the error immediately if you notice any. Equifax, Experian and TransUnion have their respective processes, so it’s best to get specific information. You can find this on their respective websites.
You can send your dispute by mail by filling out a dispute form (if necessary) and submitting a written explanation of the error. It's best to include documents that support your claim. You can send everything to the address that appears on your credit report.
However, if you find this too tedious, you can use other channels to dispute your credit report, such as by phone or online.