How can I access the U-Boot command line in my DE0-Nano-SoC board?

I have DE0-Nano-SoC running pre-compiled Linux (I'm new to the whole Linux compiling and booting - it's really overwhelming :()

I want to assign a certain size of the RAM for the Linux Kernel. I read that to do this, I must either type some commands in the U-boot command line or using the U-boot script.

My question is how can I access the U-Boot command line in my DE0-Nano-SoC board.

Thank you very much in advance.