It's fairly easy to achieve that. Follow the instructions on either of this pages:
You simply need to apply proper filter (the one from you question). Click on Tools (upper right corner). Check the checkbox in front of each ticket you want to update (or select all) => Edit Issues option and from there select Change Priority with the proper value you would like to set and confirm.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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