Acknowledging Alarms
Acknowledging an alarm indicates that you are aware of the alarm. When you acknowledge an alarm, the tag value and alarmed status of the tag is not affected, but any alarm actions or alerts such as audible tones or pop up notifications are stopped until after the alarm state has cleared.
You can acknowledge any or all alarms in the Alarm List provided you have the appropriate role (ALARM ACKNOWLEDGEMENT) to do so.
To acknowledge individual alarms:
In the alarm list, the Acknowledge column shows whether an alarm has been acknowledged. Tick the Acknowledged check box for the relevant alarm or select the alarm and click 
Note
Be careful not to tick the Acknowledged check box unless you intend to acknowledge the alarm. Depending on your system configuration you may not be able to reverse an alarm acknowledgement.
Your System Administrator can configure this behaviour by setting the Enable Confirmation of Alarm Acknowledge parameter.
To acknowledge all displayed alarms
Acknowledge all unacknowledged alarms by ticking the 
Ack Displayed Alarms button on the toolbar. If the list is filtered, only alarms currently displayed on the list will be acknowledged.
Example
For example, use the filters to display only alarms triggered before a certain date, then use the Ack Displayed Alarms tool to acknowledge only those alarms.