Disc rotors are fastened with either a six-bolt interface or a lock ring. Most modern road bikes use a lock ring - either with external notches (common with thru axles) or an internal spine (common with quick release skewers). Some bikes use both.
For a thorough walkthrough, watch this [Park Tools video].
Wear protective gloves — contaminating the disc surface will affect braking performance. Other than that, it's as simple as unscrewing a bolt. If you choose not to use a torque wrench when refitting, make sure the rotor is very firmly secured.
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