The first step is to click on Manage at the bottom left corner of SalesScreen.

A menu will pop up, where you can proceed to click on Achievements under the Gamification headline:
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Once on the Achievements management page, you will get an overview of all the Achievements which have been created and are either active or inactive. To edit an Achievements, simply click on it - or press the 3 dots and press Edit:
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You will be presented with the Achievement creation wizard, identical to the one used to create an Achievement. Edit the fields that you see necessary and click the Save changes button: