How can I decrease the value of mtu of packets sending my tor-server?

[Soviet Union]

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My mtu are lower than usual value of mtu (1500) (in accordance with
characteristics of my ISP) and I have the next ICMP from my router:
ICMP tor_node_name unreachable - need to frag (mtu 1492)

How can I decrease the value of mtu of packets sending my tor-server
for avoiding of fragmentation?

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