Hi @Anonymous , Please follow instructions from @OOlashyn . just in case if you are struggling to find the option set. Here are the steps for you.
1. once you are in the classic interface, add your choice field in the form
2. Select the choice field. Under Home > Change properties > Details Tab > press the "Edit" button
3. Then you can see option set in the bottom > click edit ( Screenshot for your reference)

Hope it helps.
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