How to restore my MacBookPro 6,2
What state is it in now? If you can access the Mac App Store, then you should be able to install High Sierra directly from there.

If you can boot to a Recovery Partition (Command R on boot), then you can erase the hard drive and do a 'clean' install of the current OS, if that is what you want to do, and then update to High Sierra from the App Store.

It's unlikely that age or software is 'slowing it down'. A full, or failing, mechanical hard drive might do that. 

The best way to keep it useful is to replace the Hard drive with an SSD, and max out the RAM.