My front valve stem has decided to start leaking

It is leaking at the base. Can it just be tightened, or do I have to replace it. Also, can I tighten it with the tire on, or does it have to be done from the inside?
Loopie
Only you can answer that ?
Is it a rubber base? Can't be tightened, need to replace. Method above.
Look at the Jake Wilson site. If it has a nut, then you can tighten it. But if the seal is gone, then you need to replace. Method again, is above.
If you are using rubber valve stems, they should be replaced every time the wheel is replaced. Preference is to go to a metal stem with a nut on the outside, as in the Wilson site.