You're going to re-design your site's UI? Awesome.
What's wrong with it? Well, you already know that, it's obvious, right?
This is where UX methodologies step in. Pre-UX design would see the designer make assumptions around what could be improved, design it, and it was done.
Now, however, UX informs UI design, giving us the pain points, the downfalls and roadblocks to what your user is trying to accomplish.
If you're making assumptions in UI design you're doing it wrong. You're not your users, so go talk to them.
a user who can’t find something on your site might be missing it for a number of reasons like the colour of the item, or the location on the webpage, the content surrounding it or the visual hierarchy it has been assigned. The problem might be multi dimensional and the only way to uncover the root issue is to conduct research.