How to create u-boot or modify existing one for 8->16mb flash upgrade on C20v1

how to create u boot for 16 mb flash ic or modified uboot.bin of 8mb to 16 mb

request u-boot source code from tp-link, it didn't come from here.

For what device ? What is the serial output with new flash?

Uboot mostly have more than one SPI size programmed. It should load kernel from the same SPI address regardless memory size.

On the other end OpenWrt has fixed memory map so if you like to have larger flash some adjustments are required

what adjustments are required to upgrade 8mb flash to 16 mb dlash can you please guide me

why are you even doing this ?
device have an USB port, you can use it to extend the flash space.

you'll fill the tiny RAM instead of the flash, but that's another issue, I guess.

i am doing to try to establishing a mesh network and this router as a mesh node and for that want to to increase flash size to install OpenWRT with mesh node

How is what I wrote earlier stopping you ?