If you have a VA loan and interest rates drop you can get always refinance your mortgage with what is known as a VA Interest Rate Reduction Loan (IRRL).
The IRRL is a pretty good deal because it makes the entire process much easier on you. Normally, to refinance a mortgage you have go through all the stuff you originally did when you got the first loan. A typical refinance requires that you give your new lender all the paperwork showing you can pay the mortgage, multiple years worth of tax returns, pay stubs, bank statements, a property appraisal and inspection that you have to pay for, etc.
The IRRL doesn't need any of this paperwork, there are also no appraisals or inspections required so you won't have any of those costs. You won't need a credit check because there is no minimum credit score requirement, however the lender will check your payment history to make sure you have been paying your existing loan as required.
An IRRL can only be used when refinancing an existing VA loan to another VA loan. You don't need to get a new certificate of eligibility from the VA for an IRRL, you can just show your lender your original one.
All VA loans do have a funding fee, however you can finance that fee as part of the mortgage, therefore you can literally get a new mortgage for no out-of-pocket costs. No lender can be required to give you an IRRL, but you don't have to use the same lender as your original loan. The VA recommends you shop around when considering an IRRL, you should also do the math to make sure refinancing is the best option for you.