My original laptop charger was output 20V 3.25A.
Now I have a charger with output 20V 6.75A.
Can I use it?
(The real data: Can I charge Lenovo Yoga 11s with Lenovo IdeaPad 15?)
Assuming they're both genuine Lenovo chargers with the same connector, the answer is most likely yes.
Case in point: my HP ENVY x360 13 shipped with a 45W adapter. It'll charge on a separately-purchased HP 65W adapter, with the higher output allowing it to charge faster than the 45W adapter when the laptop is in use. It'll also charge from any USB Power Delivery source on the USB-C port, and do so while turned on if there's at least 30W available. (It'll charge with less, but only when it's turned off or in sleep.)
The general rule is:
you can use the new power adapter.
Although this doesn't apply in your case, for laptops that charge using USB Power Delivery over a USB-C port, it's even simpler: the replacement adapter need only have an output wattage at least that of the adapter it replaces or what is specified by the manufacturer. You don't need to worry about amps or volts because the standard mandates that higher-output USB PD chargers support all voltages that a lower-output adapter would support.
In some cases, a laptop will accept a USB PD charger that is below specification, though it may discharge the battery under load because there isn't enough power available from the charger (as is the case with my HP ENVY x360 on a 30W adapter); or charge only when off or in sleep, not drawing power at all from the charger when turned on (which said laptop would do on a 15W adapter).
Yes, as long as the voltage is the same and amperage is more than required, the laptop charger will work (assuming the plugs fit)