Downloading and installing Construct 2 is pretty easy. You need to have a computer if you want to use Construct 2. You cannot run this on a Mac. You need to perform the following steps to download and install Construct 2:
Go to http://www.scirra.com.
Click on the Download button, as shown in the following screenshot:
Once you've downloaded it, follow the instructions and the installation should be simple.
The numbers refer to the version of Construct 2 you are using (https://www.scirra.com/construct2/releases). This is simply just Scirra's way of versioning the software. There are stable releases and beta releases. Scirra releases a beta version first to work out all of the bugs then they release a stable version. You should use the stable release as the beta releases might be a bit unstable. On that note, I always download the beta releases and I have never had a problem. However, it is advisable to use the stable releases.