controller.java

@PutMapping("/{studentId}")
public ResponseEntity<Void> updateStudent(
        @PathVariable Long studentId,
        @Valid @RequestBody StudentRequest request) {

    Student existing = studentRepository.findById(studentId)
            .orElseThrow(() -> new ResponseStatusException(HttpStatus.NOT_FOUND));

    updateFrom(existing, request); // or maybe more compact but less easy to read:  modelMapper.map(request, existing);
    studentRepository.save(existing);

    return ResponseEntity.noContent().build();
}

private void updateFrom(Student student, StudentRequest request) {
    student.setFirstName(request.getFirstName());
    student.setLastName(request.getLastName());
    student.setEnrolled(request.getEnrolled());
}

studentRequestDto.java

@Data
@NoArgsConstructor
@AllArgsConstructor
public class StudentRequest {
    @NotBlank
    private String firstName;

    @NotBlank
    private String lastName;

    @NotNull
    private Boolean enrolled;
}