Copter 3.5.x - location of software

I am trying localize source of copter 3.5.x software for exercise purpose. I cannot find it. Are anybody can give me some directions, please?

All you need is there…

Many thanks mlebret.

It looks I was not so accurate. Writing ‘copter 3.5.x’ I expected more versions. Not only 3.5.7
I am looking for copter 3.5.4 version in particular.

3.5.7 version is quite the same without some bugs:

Copter 3.5.7 17-Jul-2018 / 3.5.7-rc1 12-Jul-2018
Changes from 3.5.6

  1. Bug fix to ESC calibration safety switch issue

Copter 3.5.6 11-Jul-2018 / 3.5.6-rc1 07-Jul-2018
Changes from 3.5.5

  1. Safety switch fix to refuse presses once armed (BRD_SAFETYOPTION controls behaviour)

Copter 3.5.5 31-Jan-2018 / 3.5.5-rc1 24-Jan-2018
Changes from 3.5.4

  1. ICM20948 compass orientation fix
  2. LIS3MDL compass support on Pixracer
  3. support for easy embedding of defaults in firmware binary
  4. do-mount-control control command sets copter yaw when using 2-axis gimbal
  5. TradHeli fix for direct drive variable pitch tail rotors

mlebret, thank you very much again, even it is not exactly what I am asking / looking for.