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Every user can check the current app version using the 'Shift + i' shortcut when in the app. 
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Method 2 - any user
Click the "Help" button (question mark icon at top right) and select "About App" from the pop-up menu:
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Method 3 - Jira Administrator
A Jira Administrator can see the app information by going to the 'Manage apps' section and checking the details:
Expand to see app details:
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