The answers below are very good. One word of caution if switching to another lender is that you'll likely need a new appraisal. If your property was appraised at 80% loan-to-value for the purchase, it will need to do so (at least) again, in order for you to take advantage of any lower rate. If you're sure the value has remained stable or increased, you should be fine, but look closely a recent sales to make sure nothing would undermine your best intentions.
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Additionally, remember you're going to have to pay closing costs again on the new loan so that may negate the benefit you gain from a 0.5% lower rate now. You should carefully weigh the short term costs vs the longer term gains to see if it makes sense to refinance.