It is not possible to rename report columns within Standard Report Types in Salesforce.
When creating a Custom Report Type (CRT), users have the option to update the Display Label for any field added to the CRT. Once the display label is updated within a CRT, every new report created using that Custom Report Type displays the renamed labels. Refer to this article which outlines the steps in creating a Custom Report Type.
Note: Updating the Display Label in a Custom Report Type affects all new reports created from that CRT going forward. Existing reports that were already created before the label change retain the original field label names.
To rename report column labels, follow these steps after creating or identifying your Custom Report Type (CRT):
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