A demo of snmpset.
The client : nms-server2
the snmp agent (server) : router (rp-5d-2621)

The corresponding trace is set.cap
NOTE: the trace set.cap has a ping between the snmpget and snmpset to help in distinguishing the two queries.

first, let's check in the router:
rp-5d-2621#sh run | inc snmp
snmp-server community public RO
snmp-server community private RW
snmp-server contact lalabooboo <-------
rp-5d-2621#

on the NMS:
nms.server2_oansari-> snmpget 10.5.1.15 public sysContact.0
system.sysContact.0 = lalabooboo
nms.server2_oansari-> 


now let's set the value via snmpset:

nms.erver2_oansari-> snmpset 10.5.1.15 private sysContact.0 s changeit
system.sysContact.0 = changeit
nms.server2_oansari-> 


the change in the router:
rp-5d-2621#sh run | inc snmp
snmp-server community public RO
snmp-server community private RW
snmp-server contact changeit <--------
rp-5d-2621#