Need help with disabling the HDMI

Hi, i am trying to disable the HDMI but i cant do it. I know I need to edit the uEnv.txt file and there are two of them. I can access the first one but can’t seem to access the mounted one. I am trying to use this code :

1. mkdir /mnt/boot
1. mount /dev/mmcblk0p1 /mnt/boot
1. nano /mnt/boot/uEnv.txt
However, when i try to run the second line it says special device mmcblk0p1 doesn't exsit. Thanks.