There are various ways to delete customer data.
Deleting individual data
You can delete customer data stored in any field manually by removing the data element containing the target customer data. This includes documents containing customer data, such as notes and booked sales and purchase invoices.
Deleting an entire customer
You can directly delete customer data by deleting a contact and then running the batch job “Delete Interaction Log Entries for Canceled Interactions” to delete interactions for that contact.
However, you must adhere to legal retention periods. You may delete purely personal data, but not data necessary for business operations (e.g., sales history).