You can open a Blueprint Class in the Blueprint Editor either by interacting with the asset directly in the Content Browser, or by interacting with an instance of the Blueprint Class that has been placed in your level.
Content Browser
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Double-click on the icon in the Content Browser.
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Right-click on the icon in the Content Browser and under Common, select Edit....
Level Viewport
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Right-click on an instance of the Blueprint in the level and under Asset, select Edit Blueprint name.
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With an instance of the Blueprint selected, in the Details tab, click on the Edit Blueprint button.
Although these two methods involve menus attached to an individual instance of a Blueprint, please note that any changes made in the Blueprint Editor are made to the Blueprint asset. As a result, the changes will be applied to all instances of the Blueprint.