To program the board, you'll need a programmer and the relevant software.
There are a few programmer options, here is one of them:
https://www.st.com/en/development-tools/stlink-v3minie.html
The software you need is STM32CubeProgrammer:
https://www.st.com/en/development-tools/stm32cubeprog.html
Not sure what you mean by "offline", but neither of these requires an internet connection except to download the software.
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