How can I withdraw an approved I-526 form in my EB-5 application?

You can withdraw it later. No need to withdraw it now. Keep it as insurance!

You can withdraw later. They should not require an EB-5 petition to be withdrawn to process EB-2 green card.

You should be able to withdraw it before or after you become a permanent resident. Once you become a permanent resident it becomes moot anyway. Yes, the USCIS may mix up the files or ask you to clarify but as long as your EB2/I-485 have been approved, you should be fine.

From your question, I understand that you had filed three I-485s—EB-5, EB-4, and EB-2. You withdrew the EB-4 I-485 and the same has been acknowledged. Your question is silent about the status of your EB-5 I-485. Generally, an I-485 approval means other pending I-485s are automatically withdrawn. Make sure you are entitled to refund of your EB-5 investment before you voluntarily withdraw your approved I-526. Does your Limited Partnership Agreement with the Regional Center allow refunds for voluntary withdrawal?
If not, then you may face problems in recovering your investment. It is best you work with an EB-5 investment attorney to review the terms and conditions of your Agreement before proceeding to withdraw your I-526.

It all depends. Do you have your temporary two-year green card yet? If so, when will your green card expire? I think it would be best to consult with an immigration attorney before you withdraw your application.

You don't need to withdraw your approved I-526 and you shouldn't. It might become useful in the future. For example, occasionally immigrant visa petitions (such as an EB2) are revoked. If that happened then you might be able to file a new I-485 based upon the approved I-526.

You can send a letter to withdraw. When your EB-2 I-485 was approved did you get the 10-year green card? If yes, likely the I-526 is already withdrawn but best to send a letter with the copy of the receipt in any case.