I don't see how to do this via the dashboard.

It’s not possible to downgrade because we allocate 10gb of disk for the database on Pro projects and we can’t downsize. It’s something we want to fix in the future. For now, the only option is to create a new free project and move your data there.

notice that if your db is already bigger then 1gb, it won't work on a free plan

Will do. Thanks!