how can i upgrade from Mac OS X 10.6.8 version? And what is the recommended next version to upgrade? 10.7 or 10.9??

What model of MacBook do you have? That could determine your options. The latest version of OS X is 10.11.4 El Capitan. Earlier versions of OS X - 10.9.x (Mavericks), 10.10.x (Yosemite) are generally no longer available, unless you previously downloaded them from the Mac App Store, where they should be available under the Purchases tab of the Mac App Store. OS X 10.7.x Lion and OS X 10.8 Mountain Lion are still available for purchase via the Apple Store for Macs that don't support the later versions of OS X: http://www.apple.com/uk/search/OS-X-Lion?src=globalnav

Before upgrading to a later version of OS X make sure you have full backup of your system that you can restore from should you need to for any reason.

Also, although the later OS's state they will operate with 2gb RAM installed it will generally be a miserable experience with lots for beach balls to contend with. IMO 4gb of RAM is a minimum requirement and, if possible, even more if your Mac is able to be upgraded beyond 4gb.

Another important consideration is to make sure that any apps that you rely on in your workflow will continue to work in a later OS. You may be faced with having to upgrade or replace any apps rendered incompatible in the later OS. Here is a fairly comprehensive compatibility list:

http://roaringapps.com/apps

If you simply did an upgrade of the OS the process shouldn't have erased any of your documents - only the operating system (version of OS X) should have been changed, with all your data remaining intact. Losing your documents would most likely have occurred if you did an erase and install - updating the OS doesn't normally require the disk to be erased. Are you sure that you have lost your documents?

zeilrahcz86 wrote:

...But unfortunately, i had forgotten to back up all documents and its kinda lost.

would there be a possibility to retrieve all those lost files?

Guess: you've been storing your documents somewhere other than in your login directory. Maybe off somewhere in a device root directory on your boot disk? If so, those documents have been migrated off to a different folder, and you can recover those from that area, and move them back to your login directory. Look around in /Library/SystemMigration and see what you find.

Now if the system was wiped and installed, then the data is gone — many of the remnants will have been overwritten by the reformat and reinstallation process. Attempting recovery of that data won't be cheap and none of the folks that attempt this will guarantee results and — if you really want to spend that money — shut down this system right now and get a disk image to try to avoid overwriting any remnants still remaining.

If there are no backups, your own and often irreplaceable data is vulnerable. Hard disks and SSDs will fail. Computers will fail. Computers will be damaged or lost or stolen. This is inevitable. When you have no backups, your data is at risk from all of these causes, as well as from command mistakes and software bugs. Get Time Machine or other backups going right now.