To free up storage space in LimeSurvey, you can delete responses and their uploaded files. Here are the key steps:
Open your survey's Results tab and go to the Responses list. Before you delete anything, select the responses you want — tick their checkboxes to act on several at once — and use Download files to keep copies of any uploaded files, because uploaded files are deleted together with their response and are not included in exported survey archives (*.lsa files).
To remove only the uploaded files while keeping the responses, choose Delete attachments; to remove the responses entirely, choose Delete.
After removing data, navigate to Configuration → Global, select the Storage tab, and click Calculate storage to refresh your storage counter. Wait a few seconds and reload the page to see the new storage count.