how to get sidebar from right to the left side

i have a theme with a sidebar at the right but i try to get it to the totally left side. i can’t get it fixed. i don’t understand it. how can i get my sidebar from the right to the left side. my site: https://www.ditkannietwaarzijn.nl

i hope someone can help me. i work with the 3c-black-letterhead theme. i managed to get the leftside out of it and now hope someone knows how to get the sidebar at the right to the left. can anybody help me?