Changing default Nav menu buttons to my own

Hi all!
Just need some help here. Has me stumped.

I need to change the exisitng 3 Navigation Menu tab/buttons to 3 of my own which I have. These are up and over buttons, so I have 6 in total.

I have looked everywhere on how to do this but none have made enough sense and each say various different things.
I know I perhaps need to do something with unordered lists?? I dont know this stuff. My coding skills are okay to bad. I can do whatever though as long as it is explained well.

If anyone knows how to do this or can point me in the direction if a good tutorial that would be amazing! I thank you.

here is a link to my site. I am using Suffusion theme.
https://www.rerdownloads.com