Depending on the bike, it's a simple matter of getting access to the back of the gauges. There are two bulbs in each gauge (in my my custom's anyway!). As for the tail-light, they have double filiment bulbs. One lower wattage for the 'running lights' and a higher wattage filiment for the brake light. If either is extinguished, the bulbs should be replaced.
Imho, once you have the covers off, I would go ahead and replace all the bulbs at the same time - as they are likely to all expire in the near future if you don't.