UpdateRelatedContact.trigger

trigger UpdateRelatedContact on Account (after update) {
    Map<Id, Contact> contacts = new Map<Id, Contact>();
    for(Integer i = 0, s = Trigger.new.size(); i < s; i++) {
        Account oldRecord = Trigger.old[i], newRecord = Trigger.new[i];
        if(
            oldRecord.Field1 != newRecord.Field1 ||
            oldRecord.Field2 != newRecord.Field2 ||
            oldRecord.Field3 != newRecord.Field3
        ) {
            contacts.put(newRecord.Id, new Contact(
                Field1 = newRecord.Field1,
                Field2 = newRecord.Field2,
                Field3 = newRecord.Field3
            ));
        }
    }
    for(Contact record:[
        SELECT AccountId 
        FROM Contact 
        WHERE AccountId = :contacts.keySet()
    ]) {
        contacts.get(record.AccountId).Id = record.Id;
    }
    upsert contacts.values(), Contact.Id;
}