Use librem5-flash-image Repo in Dev Kit Flashing the System Image How To Guide
This section is still saying to clone from https://source.puri.sm/Librem5/librem5-devkit-tools.git when it should now be from https://source.puri.sm/Librem5/librem5-flash-image.git since this script has moved on June 30th.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information