Q: How do I re install the OS El Capitan after I upgrade to ssd on a mac book pro 2012? I have a time capsule with back up.

Make a USB flash installer:

Create a bootable installer for OS X - Apple Support

Install the SSD. Format and install OS X. Use Migration Assistant to get back all your prefs and applications (and other files if you wish) - using either the old drive connected externally, or using TIme Machine as the source.

A real easy way would be to get an external enclosure for the new SSD, plug it into your Mac, format it correctly, and make a bootable clone copy of your entire system with CarbonCopyCloner or SuperDuper. Then swap the two hard drives and you're done - since it's formatted and a bootable clone, there is nothing more to do than boot up with the new SSD.

Get Carbon Copy Cloner and and a USB case for your new SSD or a USB adapter.

Clone it directly to the new SSD and your done.

Did it many times works like a charm.

Not sure why you are recommending that to me? That is exactly what I had suggested to the OP.

Mea culpa, I need new glasses.