As a secondary option, this below link is the direct checkout link that I used.
You can change to a free account immediately as soon as the premium account is activated to never be charged for premium. This does not start a refund, the premium account will still be valid for the full remaining period until the date listed. For example, I purchased this today on 22/04/20, it states during and after the change "Your plan will change to Spotify Free on 21/6/20".
On a PC, login to spotify, goto Profile > Account > Select Free Plan.