Q: how to download os X el capital when macos sierra is out?

Hi,

It depends on your circumstances. It can be difficult to find old OS downloads.

If you purchased it previously it will be in the App Store under the Purchased tab.

If your mac came with it pre-installed you may have to go the internet recovery or Recovery HD route, but that would entail wiping your hard drive in order to do the install. You could then migrate your user data from a Time Machine backup or similar.

Alternatively, if you have a bootable backup of your El Cap system (assuming you had it installed) you could reinstall that.

More specific answers will require more specific information from you (it's generally a good idea/helpful to provide more information so that answers can be geared towards your situation and others in the same situation can benefit).

Whatever you do, make sure you have a full working backup (which you have tested) so that you can revert should something fail.

Ok, so you'll need to download El Cap from here: Download OS X El Capitan 10.11.6 Combo Update

And burn it to a USB key like so: Create a bootable installer for OS X - Apple Support
Then boot on your key (press alt when booting) and install

There is likely no place you can get El Capitan.

There is a version available if your Mac cannot run Sierra or you have Snow Leopard installed and want to upgrade to Sierra (apparently, you cannot upgrade directly to Sierra from Snow Leopard).

Upgrade to OS X El Capitan - Apple Support

Since your Mac shipped with 10.7 (Lion), there is no way you can get El Capitan using that Mac.

If you have a Mac running Snow Leopard, you could download the El Capitan installer on that Mac and transfer the installer app to your MacBook Pro.

And, no, you cannot make a bootable installer from the Combo Update to 10.11.6. It is not the full OS, just an update.

Should I consider upgrade to Sierra? Its been out just about a month?

You should evaluate the software and hardware you use with your Mac and make sure it is compatible with Sierra or can be updated to be compatible. If you cannot run the software you need on Sierra, it wouldn't be advisable to upgrade. Otherwise, I see no reason to hold back.

It is a pretty giant leap to go from Mountain Lion to Sierra, so there are things to consider.

I can't remember everything that has changed from that far back, but here are some biggies:

• Some people are dead-set against using the iPhoto replacement Photos. If you use iPhoto and your livelihood depends on the one or two minor features left out of Photos, then you need to make sure you update to version 9.6.1 of iPhoto in the App Store.
• All of the Libraries (iPhoto, Mail, iTunes, etc) are updated to new versions, so reverting back is difficult without a backup. So, backup before upgrading.

As to performance, you can install Sierra on an external hard drive or even 16GB (or larger) USB stick. While running from an external drive will be slow, you can get an idea of the changes and performance hits.

What you might consider is replacing the spinning hard drive with an SSD. Buy an enclosure for you internal hard drive and use it as a backup or extra storage. Given it's age, though, maybe a backup wouldn't be the best use of it.

Apple has made it easy to upgrade to El Capitan for those who are unable to upgrade to Sierra.  There is a link on that page to El Capitan on the App Store for those who had never downloaded it before.

Upgrade to OS X El Capitan - Apple Support

As you noted, you must have access to the App Store to download it, so there is also a section on what to do if you don't have the App Store on your OS version.