Short answer
Install the free Nextcloud app from nextcloud.com/install, then sign in with your storage address, username and password (from your welcome email or the client area under Services).
Computer
The Nextcloud desktop app keeps a folder on your computer in step with your storage. Choose which folders to sync if your computer is short of space.
Phone and tablet
The Nextcloud app for Android and iPhone lets you browse your files and can upload new photos automatically.
In a web browser
Sign in at your storage address to upload, download and share files from any browser.