The nanoVNA includes STM32 bootloader firmware for replacing nanoVNA firmware.
This bootloader firmware gets control by powering on a nanoVNA with P1 contacts connected:

While some attempt temporarily connecting P1 contacts using e.g. tweezers, less coordinated among us prefer a switch.
Clipping either outer pin of a SPDT switch with compatible pin spacing results in a suitable SPST switch:

The back cover needed notching for clearance:

Firmware, DFU utilities and downloading instructions can be found on Hugen's Google Drive.