Will reseting my 2011 Macbook Pro bring it back to the old version?
If you Mac was shipped with OS X 10.6 or earlier, then the Mac came with an installation DVD. If your Mac was shipped with OS X 10.7 or later, then you can download that version from the internet. You will need to boot your Mac using Internet Recovery Mode and download the original shipped OS X version. 


  Early 2011 MacBook Pro models shipped with OS X 10.6 and the late 2011 MacBook Pro models came with OS X 10.7 installed.


Snow Leopard OS 10.6 is now to old to be of any use for accessing the internet or running most current applications. OS X 10.7 is better, but not by much. Your best bet is to see what previous purchased versions of OS X are still available from the Apple Store. I believe, Yosemite OS X 10.10 can not be downloaded even if you previous purchased this version.

It is generally a good idea to backup any version of OS X you download, before installing or upgrading your operating system. What I mean by this is that you backup the .dmg file you downloaded. What people are learning is that you can not rely on Apple to allow you to download a previously purchased OS X version at a later date.

If you can download Mavericks OS X 10.9 or later, you can copy the installation files to a flash drive. You can boot from the flash drive and perform a clean install of OS X. The instruction are given at the web site Create a bootable installer for OS X.