Shoud I uninstall mongodb 2.6 before upgrade to 3.0? I installed mongodb 2.6 in my machine. This week, I followed the instructions in the subsection of Install MongoDB on http://docs.mongodb.org/manual/tutorial/install-mongodb-on-ubuntu/. I installed MongoDB 3.0/. But I found that the date of some files in the mongodb package is in Feb. Why? Should I un-install mongodb 2.6 before upgrade to 3.0? my cluster is running all the time....
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.