Apple has recently released a new firmware update for its Pro Display XDR. The software includes a calibrator for in-field recalibration as part of its 4.2.30 version update.
Aside from the calibration tool Apple offers a support document that can walk users through how to calibrate their Pro Display XDR, with comprehensive details about fine-tuning and measuring via preferences and in-field recalibration.

In-field recalibration allows third party spectroradiometers to calibrate the Pro Display XDR, which was only introduced in the latest firmware update. The requirement to do so is a supported spectroradiometer, the device update and a computer running macOS Catalina 10.15.6.
Apple has also released resources to help Pro XDR Display users get the most of their displays. Some of the most helpful ones include cool ambient temperature calibration, spectroradiometer setup and ambient lighting control.
The firmware update, version 4.2.30 should be available to download today on Apple’s official website.