Upgrade Mac OS X El Capitan version 10.11.6?

The next and final upgrade for your Late 2009 iMac is High Sierra 10.13.6. Visit the How to upgrade to macOS High Sierra page using only the Safari browser. There will be a link on that page to the appropriate High Sierra download in the Mac App Store. I recommend making a final Time Machine backup before the upgrade.

I recommend your doing this before Apple releases the macOS 11 (Big Sur) operating system, and you will still receive Security and browser updates (e.g. Safari 13.1.2) in the Mac App Store's Updates panel. Once Apple releases Big Sur, the High Sierra operating system drops off the customary three-year sliding window of support.

Press and hold the option/ alt key while opening Photos, a Choose Library dialogue

will open listing all the photo library you can use with Photos, this will include iPhoto

libraries. Select your iPhoto library and click on Choose Library, this will now import

the contents of your iPhoto library into Photos.

iPhoto as an app does not take up any space on your mac, it is the iPhotos library where your images are that takes up the space. Now that you have imported your images from your iPhoto library into Photos your iPhoto library will now be

named as photos library or similar. Do not delete this as you will lose all your images.

To check look in your Pictures folder.

Apple has said "this Fall," and it based on that vagueness, it could drop at any time.