WordPress theme update

I'm looking to move my website to a new theme, but I want to make sure that all the old crap from the old theme is removed too.

I have my own dedicated server with the new theme installed, I need all my old content, images, and any other relevant data moved.

I tried myself with the import / export tool, but the images kept timing out due to being around 6GB in total. I'm not looking for any other development, just a clean copy of my own site, with the new theme and old content.