An I-94 is not a visa --- it is essentially an expiration date. An I-94 document contains the designated time you are allowed to stay in the United States on a single trip, and it is placed in your passport when you enter through U.S. customs. It is a nonnegotiable document, and there is nothing anyone can do to alter the expiration date. In this sense, a "renewal" of an I-94 is not possible. However, it is possible extend your stay in the United States by obtaining a replacement I-94 through visa extension or change of status.
Download, print and complete Form I-539, Application to Extend/Change Nonimmigrant Status, from the website of the United States Citizenship and Immigration Services. You must complete and submit the application at least 45 days before your I-94 expires. You can submit Form I-539 electronically through the USCIS website if you prefer.
Gather the supporting documents. Along with a completed Form I-539, you must submit proof of financial support in the United States during the extended stay, a letter stating the reason for the extension request, a copy of a future return ticket, if applicable, and photocopies of your passport information and of Form I-539. You must also submit the filing fee of $290 (as of March 2011) in the form of a check or money order made payable to the U.S. Department of Homeland Security. If filing Form I-539 electronically, send all supporting documents in the mail with the printed e-filing confirmation page. If you are changing status from one nonimmigrant visa to another, you may be required to submit additional documents.
Send your application and supporting documents to the appropriate USCIS filing center. The filing location depends on your particular circumstances and visa type. Refer to the instruction packet for Form I-539 under "Where to File" to locate the appropriate address. If your extension request is approved, you will be given a new I-94.