If you have Q in your channel, use the settopic command
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Sets both the current and default topics on the channel to that specified. Note: To use this command you must have +t on the channel.

Usage: /MSG Q SETTOPIC <#channel> <topic>
eg. /MSG Q SETTOPIC #[QN] This is a GREAT topic smile


And Chanflags
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+f - Force topic
Prevent people change the topic with /topic command - SETTOPIC is needed.