Synology DiskStation Manager 7 (DSM 7) was released on 29th June 2021. Right now, the upgrading process is still manual. You have to go to Synology Download Center, input your Synology NAS model, and choose your current DSM version to upgrade from.
I upgraded both my DS920+ and DS1621+, and my current DSM version for both is DSM 6.2.4. After clicking download, it will prompt you to download a .pat file. The file size is about 340MB.