In ERPNext, you can change fields, selection values in a DocType by using the Customize Form tool. This tool allows you to insert Custom Fields as per your company's specific requirements or customize properties of standard fields.
To access this tool,
1. Go to Settings > Customize form. You may also simply search "Customize Form" on the search bar.
2. On the Customize Form page, select the Form Type you want to update.
3. After selecting the Form Type, this will expand the page and list all the standard fields in the form.
4. On the Fields table, you may simply click the arrow on each row to update the values.
5. The Field information pop up will appear. Here, you can make the necessary changes in the data field such as label, selection and data type.
6. Click Save to apply the changes.