Do it one instance at a time.
Stop the instance dependencies from cluster management.
Change the password in services.msc
start the instance from cluster management.
Repeat the process for the other instance.
I recommend stopping the instance so that the service does not switch to the other node while changing the password.
Jason...AKA CirqueDeSQLeil
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I have given a name to my pain...MCM SQL Server, MVP
SQL RNNR
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