If you are still eligible for employment authorization under the same immigration category that you applied initially, and your EAD has already expired or is expiring soon, you must submit to USCIS a new Form I-765, Application for Employment Authorization, and pay the stipulated fee (if applicable).
Generally, you should not file for renewal of your work permit more than 180 days before your original work permit expires. Ideally, you should apply to extend your EAD card within 3-4 months prior to the expiration date stated on the front part of your current EAD card. Please note that you should submit your application at least 90 days before the expiration date, since it may take that long for you to receive a new Employment Authorization Document.