To dispute Experian you must compose a dispute letter and mail it to them. Upon receipt of your letter they will investigate the item.
The first step though is to get a copy of your credit report. You can get a free credit report once a year from each credit bureau. I suggest you go to annual credit report, they will provide you with your free copy.
When you have your credit report you must identify what listings are inaccurate or incorrect. These will be the listings that you dispute.
These marks are disputed by writing a dispute letter and sending it to Experian. Upon receipt they will say if your letter is valid or invalid.
If they find your letter invalid you will receive a letter from them asking for more details about the item. You should respond accordingly and provide them with the information.
However if they find your letter valid they will investigate the item. During an investigation they will contact the originator of the item and ask them to verify the account, the balance, and the dates on the account.
It has been found that investigations will often result in a listing being removed. This is a result of many businesses being unwilling to spend the resources verifying disputed debts.
Your other option is to hire a credit repair service. If you do hire a service all you will need to do is identify what items you wish to dispute.
The advantage of having a service is in case the listing is verified they have advanced dispute techniques they can use. These include; escalated dispute information requests, debt validation, and creditor direct intervention.
If you only have minor damage on your report then I suggest repairing your credit yourself. However if you have multiple negative marks on your report I suggest a service. They can also help getting a valid letter submitted to the bureaus.
You are going to need to dispute a bad credit item with each credit bureau. This means you will have to send a separate letter for the same item to each bureau.
In sum you can remove bad credit items from your credit report. To do this you must dispute the item with each credit bureau.