The Callback Class

//The enqueue deployment call (last call above), can optionally take a callback argument.
//I was running into an issue and needed a bit more debugging, so here's my callback for that.  
//This call can be an inner class, by the way.
public class CustomMetadataCallback implements Metadata.DeployCallback {
        public void handleResult(Metadata.DeployResult result, Metadata.DeployCallbackContext context) {
            System.debug('status: ' + result.status);
            if(!result.success){
                System.debug('error1: ' + result.errorMessage);
                System.debug('error2: ' + result.errorStatusCode);
                System.debug('error3: ' + result.errorMessage);
                System.debug('error4: ' + result.numberComponentErrors);

                for(Metadata.DeployMessage msg : result.details.componentFailures){
                    System.debug('msg problem: ' + msg.problem);
                    System.debug('msg problem type: ' + msg.problemType);
                }
            }
        }
    }

Update CMD Apex

        Metadata.CustomMetadata settings = new Metadata.CustomMetadata();
        settings.fullName = 'Your_Custom_Metadata_API_Name_WITHOUT_UNDERSCOREUNDERSCOREMDT_GOES_HERE.Default'; //Default is the name on 1 of many of my custom metadta type records
        settings.label = 'Default'; //Default is also a label. and you will need the label for the update to succeed.
        
        //Only updating one field.. if you need more fields updated, just instanciate more CustomMetdataValues and add them to the values list at line 9.
        Metadata.CustomMetadataValue dateValue = new Metadata.CustomMetadataValue();
        dateValue.field = 'Current_Sales_Date__c'; //The field I'm updating
        dateValue.value = Date.newInstance(2019,3,8);  //Value can be anything, really, but I needed a date.
        settings.values.add(dateValue);


        Metadata.DeployContainer mdContainer = new Metadata.DeployContainer();
        mdContainer.addMetadata(settings);
        Metadata.Operations.enqueueDeployment(mdContainer, new CustomMetadataCallback());