Cast Dispel on the Target of the Magic Jar
The target of magic jar is always in the jar or possessing a body. Wherever that is should be the target for dispel magic.
In the description of the spell, the target of the spell is referred to as "you". Wherever that "you" is, is where the target of magic jar is, and should be the target of dispel magic.
Initially in the Container
The "you" is in the container.
Your body falls into a catatonic state as your soul leaves it and enters the container you used for the spell's material component... The only action you can take is to project your soul up to 100 feet out of the container, either returning to your living body (and ending the spell) or attempting to possess a humanoids body.
Possessing a Host Body
The "you" is in the host body.
While possessing a body, you can use your action to return from the host body to the container if it is within 100 feet of you, returning the host creature's soul to its body