How to edit u-boot's device tree?

Hi I am using 10AS057K4F40E3SG & Quartus pro 18.1.
Also I am using HPS emac1 and marvell’s 88e1512 PHY for KERNEL ethernet.

I want to change this PHY chip’s mode. After kernel boots completely, ping test with my PC works well.
It means mode changed that I wanted. (I changed kernel source drivers/net/phy/marvell.c --> PHY mode variable.)

But When I boots u-boot only, ping test with my PC fails. I asked this issue on the intel forum.
Someone said that I should change the u-boot’s device tree.

But I don’t have any information about u-boot’s device tree location and how to edit device tree.

Do you have guide doc. or links about how to edit uboot dts especially for marvell device??
And where is the u-boot’s device tree?