BeagleBone Black & System variables


I connected a device to the beaglebone black (DLP® LightCrafter™ Display 2000). This device needs to have a variable set every time I turn it on. The command for that is “export DISPLAY=:0”
I know that I can set this system variable as a default value during initialization. And I know that I can set these in /etc/profile or /etc/profile.d. However, I cannot edit neither of the files. The permission is denied.

Please, could you help me to set this variable?

Thank you,