Update 1.4 added many new items to the Stardew Valley, including the Golden Scythe. It is much more powerful than a normal scythe, and allows you to collect more grass in a shorter time.
To unlock a new tool, you need to access the new mine, which is located in the mine. Inside the Mining Quarry, look for the entrance to the mine and start exploring it.
The new mine has only one level, at the end of which is a statue with a scythe in hand. Keep in mind that the mine is full of new enemies, so I recommend you to take a few healing items with you. You will also need a good sword.
To get the scythe, make sure you have room in your inventory and then interact with the statue.