Obtaining the source
The source of the U-Boot project is maintained in a Git repository.
You can download the source via
git clone https://source.denx.de/u-boot/u-boot.git
A mirror of the source is maintained on Github
git clone https://github.com/u-boot/u-boot
The released versions are available as tags which use the naming scheme:
v<year>.<month>
Release candidates are named:
v<year>.<month>-rc<number>
To checkout the October 2020 release you would use:
git checkout v2020.10