Given the information and by looking at the site it’s hard to know exactly how you can add the logo. If you can give us the theme link/documentation then it will be much easier to know the exact solution. But with my years of knowledge, here are the best possible solutions.
- You should check “Appearance > Customize”, there you might see any settings to add a custom logo.
- If there are any “Theme Options” for this theme in the backend.