I'd imagine your config-register to boot into flash would be 0x2102 like most Cisco routers. To clear the config, issue the command in priveleged mode "erase nvram:" or "write erase" (then reboot). This link gives a template for configuring an 837, however just remember that these are still Cisco routers. Most of the commands and configuration tasks will be the same as say a 2600.
The administrator has disabled public write access.