where is uboot source code present imx8mplus evk

[srinivas_123]

i am using IMXLXYOCTOUG v.LF6.6.3_1.0.0 version , machine is imx8mplus lpddr4 evk . 

i want to know where the uboot source code is present . i want to change the DDR rate , and I want to know to Rebuild U-Boot  

where is the Uboot source code ?

how to Rebuild the the Uboot ?

as of now , I am directly flashed the wic.zst file into sd card and it is booted up.

but now I have to change uboot source code and I don't know what I have to do ?

[Harvey021]

You can refer to the section Porting U-Boot> from the guide IMX_PORTING_GUIDE.pdf  or the section <4.5.13 How to build imx-boot image by using imx-mkimage> from the guide 

IMX_LINUX_USERS_GUIDE.pdf 

Regards

Harvey