Unfortunately, backend access is required in order to adjust the collection level weightings. We currently have the development of the user interface for this feature on the roadmap and it will be available in the near future.
The good news however, is that it should be relatively simple someone with backend access to do. Please reach out to your corresponding support team and I'm sure they can help.
Please also check out this knowledge base article on upweighting and downweighting sections of the site. Depending on your needs, it might be suitable.
Hope this helps.