You need to be root to do that. Either use sudo to run the command or su to root inside a shell. I usually do the latter as I find it more convenient but your machine may be different. For sudo use a command like su 'yum update' or similar and you have to supply the password. If you just want to su inside a shell then type su - at the command line and supply the password then change directory to /etc/yum.repos.d. Whatever you do, do not alter the permission bits on the directory as that's a security hole.
(Registered Linux User #405603 :) )