How to delete a hard-drive partition on a Mac
It's time to remove the partition you created for the public beta of OS X El Capitan.
If you got a head start with OS X 10.11 El Capitan this summer and followed my advice to install the public beta on a partition, then it's time to reclaim some hard-drive space by getting rid of that partition now that Apple has released the final version of El Capitan. Deleting a partition is a simple, two-step process using Disk Utility, which received an overhaul with El Capitan.
Step 1: Erase partition
To get started, open Disk Utility by searching for it using Spotlight or finding it in Applications > Utilities.
Next, click on the partition that you want to delete in the left panel.
With your partition selected, click the Erase button at the top of Disk Utility. This will bring up a window with an Erase button. Click Erase and then click Done.
Step 2: Remove partition
With the data erased from your partition, you still need to delete the now-empty partition in order to reallocate its space to the rest of your hard drive.
To do so, select your hard drive from the left panel and then click the Partition button at the top of Disk Utility. On the pie graph on the left, click the wedge that represents the partition you just erased. Click the "-" button and then click Apply.
After a minute or so, Disk Utility will finish applying changes to your drive and deem the operation successful. Click Done to complete your work here.
For more, get all you need to know about OS X El Capitan.