Get alerts on item changes in SharePoint Online
You can get an alert whenever a file, link, or folder is changed in a SharePoint Online document library. Depending on the item (file, folder, link), you may see different options when you set an alert.
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Open the document library and hover over the file, link, or folder for which you want to get an alert, and click the check mark.
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Click the Files Tab from the top left Ribbon, and then click Alert Me.
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In the Alert me when items change dialog, select and change the options you want. For settings and descriptions, see Alert Options below.
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Click OK when done.
Get alerts on all changes in a document library in SharePoint Online
You can get an alert whenever any file, link, or folder changes. Depending on the item (file, folder, link), there are a few different alert options.
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Open the SharePoint Online document library and make sure no items are selected.
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Click the Library Tab on the Ribbon and then click Alert Me.

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In the Alert me when items change dialog, change or fill in the options you want. For settings and descriptions, see Alert Options below.
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Click OK when done.