Shoud I uninstall mongodb 2.6 before upgrade to 3.0?
No its not necessary to uninstall. 
I don't really like yum or apt for installing mongoDB. I suggest you to download the binaries from https://www.mongodb.org/dl/linux/x86_64 , linux/mongodb-linux-x86_64-ubuntu1404-3.1.0.tgz (or the v12) and use it to start mongodb.