The 777 is Boeing's big twin jet wide body airliner. It's been flying for 20 years. Boeing plans to keep it flying for quite a bit longer. They have announced a series of upgrades, each one minor in itself, that will cut fuel burn by 2%. And they plan to pack in another 14 seats, so the fuel burn per seat might go down by 5%. Which doesn't seem earth shaking, but every little bit adds up. The 777 can hold 40,000 gallons of fuel, about four 18 wheeler tank truck loads. Saving 2% on a long flight might be 800 gallons. At $2.50 a gallon, that's $2000. Per flight.
One improvement is to remove the tail skid, and take out some beef in the aft fuselage designed to carry the loads from the tail skid, should the tail drag on the runway during landing or takeoff. They souped up the autopilot to absolutely prevent tail dragging. Boeing of course claims the improved autopilot will be infallible.
There are a bunch of other things, to numerous to list, and not all that interesting, unless you are out shopping for wide bodied airliners.