Reformatting 2011 Macbook pro and updating software...

Hello, so this is my first time on the discussion forums, and I really hope I post this question in the right place.

I have a late 2011 Macbook Pro.

2.4 GHz Core i5 processor.

4 GB 1333 MHz DDR3 memory.

I have 208.42 GB free of 498.88 GB.

I am currently running OS X Yosemite 10.10.5.

My Mac has been running really slow lately and is also extremely slow when booting. Photography is a hobby of mine, which takes up most of my memory. I have an external hard drive I am beginning to back everything up to. I was thinking that I could reformat my Macbook and then upgrade to El Capitan. I just wanted to check with people who have reformatted in the past.

Should I continue with the reformatting? Should I reformat and this update the software or should I update and then reformat?

Again, I really hope I posted this in the right place and I apologize if I didn't!

Thanks so much!