Can I salvage undercooked flan
I haven't tried that, but in theory, a standard custard should be capable of rebaking. However, it will be a tricky matter, much trickier than the first baking. 

For example, the temperature at which certain proteins in the egg coagulate depends on the speed at which the egg is heated. As a side effect, it is much harder to get a good custard starting with fridge-cold eggs than with room-warm eggs. Bringing the flan to room temperature first would not be safe (and no, reheating won't make it safe again). Second, they undergo changes not only during heating, but also during the subsequent cooling, and this may interfere a bit (I am conjecturing here - maybe it won't cause problems, but it is possible that it will). As a result, you will have a much narrower window between not-yet-ready and curdled. If the recipe contains acid, it will be even more problematic. High sugar levels will alleviate the problem a bit. What is even worse is that we are talking oven custard here, where you have much less control than on stovetop. 

If I were in your situation, I would consider the experiment worth doing. I would be extra careful (water bath and bake by temperature, probably aiming at 83 celsius, less if the custard is thick). If you are inexperienced with custards and don't have a thermometer, it may not be worth the risk for you, after all a semiliquid custard is tastier than a curdled custard. But if you do it, don't forget to tell us how it went, as I am curious. 

The above assumes a pure custard (egg yolk, dairy, sugar, taste modifiers or fillers). If you also used starch or flour, then it is too late to save it. This type of custard needs to be heated to very high temperatures (above 96 celsius) before an enzyme in the egg yolk converts the starch to goo. No sense in rebaking, but it can be reused. 

If you decide not to rebake, there are countless applications for a thick-but-not-firm custard. If you don't want to consume it pure (and this is a viable option - sometimes it is made on purpose, as in vla), use it as a substitute in any recipe calling for creme angalise. You can dilute it with small amounts of cream if the recipe needs a more pourable consistency. If you decide to dilute, do it in small steps. You can also use it instead of creme patisserie, but without dilution.