A few months before your term ends, your current lender will mail or email a renewal offer with a new rate already filled in. It's easy to sign it and move on — most people do. It's also usually not the best rate available to you, because your existing lender is betting that inertia is worth more to them than competition is to you.
Why the first offer usually isn't the best one
Retaining an existing customer costs a lender less than acquiring a new one, but that saving doesn't automatically get passed to you unless you ask, or unless another lender is competing for your business. Renewal offers are frequently priced a little higher than what the same lender would offer a new client walking in the door with the same qualifications.
What actually changes at renewal
You're re-pricing the same outstanding balance at a new rate for a new term — nothing about your amortization schedule resets unless you choose to extend it. That makes renewal one of the few moments where a rate difference of even a few tenths of a percent is easy to calculate precisely, because the balance and remaining amortization are already fixed.
What to actually do about it
Start a few months early — most renewal offers arrive with more lead time than people use. Get a comparison quote before you sign anything, even if you end up staying with your current lender; knowing what else is available is what gives you the leverage to ask for a better number. And read the term itself, not just the rate: a shorter term at a slightly higher rate can be the better bet if you expect rates to fall before your next renewal.
Use the Renewal calculator to see exactly what a lower rate is worth on your specific balance and term before you decide.