El Capitan -- want to increase partition of SSD -- want to get rid of Elementary OS

I had to reset the PRAM on my MAC, and now I'm not able to boot into EOS. I've been asking questions about how to get to do this, but i'm new to Linux.

Anyway, I didnt have much customized on the EOS partition, so I'm thinking that I'd just like to repartition the whole disk back to El Capitan and start over. I assume to use Disk utility, yes?

If I do that can someone instruct me how, and will I lose any of my current data on the Mac partition?

I have 2 recovery mac partitions, and the EOS partition including another linux swap file.

I've backed up my disk by the way, this morning.

Yes, you use Disk Utility, but you have to be booted from some other drive, you cannot partition the boot drive.

And, yes, you will lose EVERYTHING on the drive.

To do what you need, read this: https://support.apple.com/en-us/HT201314

If you have a recovery partition, the Command-R will work. IF you don't you'll need to use Option Command R to get an internet recovery underway, which will take quite a while as it has to download the operating system from the Internet.

NOTE: It says "hold down" the keys. It takes quite a while holding them down before you see that it has started the Recovery or Internet Recovery process. Keep holding the keys down until you see progress.