From the snmpd.conf man page:
The Net-SNMP agent can be instructed to re-read the various configuration files, either via an snmpset assignment of integer(1) to UCD-SNMP-MIB::versionUpdateConfig.0 (.1.3.6.1.4.1.2021.100.11.0), or by sending a kill -HUP signal to the agent process.