Mac OS X 10.10 is now available to download, but what happens if it is not ideal for you to download, such as if you need to install it on multiple devices, running a clean install or any other reason? The solution is to use a bootable Yosemite USB installer, which is not as hard as you may think, all it requires is that you get a copy of the new operating system onto a USB Flash drive.
Then easiest option is to burn OS X 10.0 Yosemite onto a disk by using DiskMaker X, which is a free app. The only issue is that you will have to download the beta version, as the full version is not supported by Yosemite at this time. You will then need to proceed to insert an 8GB (minimum) USB Flash Drive into your USB port, and it is at this point you will need to make certain any important files have been backed up.
You will then want to run the DiskMaker X program and select Yosemite from the list of options, and the software will do the rest. Once completed you can then insert the USB drive into the Mac in need of the OS update and press the Option key while booting up.
If you do not wish to install this software, you can use the Command Terminal instead, details of which can be found in the video below. It is from the beta version of Yosemite, but the process is still the same.