The flash IC is located at the bottom center of the mainboard. Sadly, access to the IC is blocked by the magnesum frame, so you need to disassemble the entire laptop and remove the mainboard.
Below is a picture of IC on the mainboard, with the pinouts labeled.
The chip will either be a Macronix MX25L6404E (shown above) or a Winbond W25Q64CVSIG. Do not rely on dots painted in the corner of the chip (such as the blue dot pictured) to orient the pins!
For more details have a look at T420 / T520 / X220 / T420s / W520 common and
Steps to access the flash IC are described here T4xx series.