How to read/write a STM8S?

You'll need software that generates a binary to load (ref STVD-STM8 at https://www.st.com/en/development-tools/stvd-stm8.html, sdcc at https://sdcc.sourceforge.net/, or asst8 at https://shop-pdp.net/ashtml/asxxxx.php).

You'll also need the programmer hardware that sends the binary via the SWIM port (ref st-linkv2 at https://www.st.com/en/development-tools/st-link-v2.html), and the software to drive the programmer, to upload your generated binary (ref ref STVD-STM8 at https://www.st.com/en/development-tools/stvd-stm8.html or STM8-Flash at https://github.com/vdudouyt/stm8flash)

Alternatively you can use a dev board, which has the chip and the programmer both on the board.