Plug the new switch into an existing connection, and give it a good 45 minutes to upgrade its software. It must be running the same software as existing stack members. Hopefully your existing stack has a full "ring". Break the ring, and plug in your new switch, and re-form the ring. Within maybe 5 to 10 minutes if you go to "Switch/Switch Stacks". It will report there is an additional member than can be added to the stack. Let it add it to the stack. You are done. Note that depending on the code version they have be a short disruption to service while you do this, so do it out of peak time.
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