On the transfer list locate some large transfers that you don't need anymore. Note: only permanent transfers and transfers sent after 16.03.2020 are counted towards your storage use - deleting these will allow you to get some more free space. But deleting other transfers (non-permanent sent before 16.03.2020) will not affect your storage usage.
Click the transfer you wish to delete to load its details.
Click the "Delete" button to the right on the transfer toolbar.
Storage space is updated immediately - you can see it on your profile.