You can re-use a COE if you've sold the home you bought with the prior loan and have paid that loan in full among other requirements.  If you wish to request an entitlement restoration, fill out a Request for Certificate of Eligibility VA Form 26-1880 and send it to the VA regional loan center for your state.
What are the requirements for re-using a COE?
Have you sold the home that you bought with the prior loan and paid that loan in full?
I have not sold the home.  I was wondering if I could keep it and use it as a rental property.