Nordic Thingy:91™ has two buttons.
The main button, located underneath the Nordic Semiconductor logo, is termed as the SW3 button and it is used for user input. It is connected to the nRF9160 System in Package (SiP). The second button, termed as SW4, is connected to the nRF52840 System on Chip (SoC). It is accessible only when the rubber cover on the device is removed. Refer the image Getting started with Thingy:91 to locate the buttons.
Either of the two buttons, SW3 or SW4, can be used to activate the serial recovery mode of Nordic Thingy:91 to update the nRF9160 SiP or the nRF52840 SoC respectively.
For more information, see Updating firmware through USB.