Warning: This question is based on poor/immature understanding of instantons and the vacuum structure of the SU(N) Yang-Mills theory.
Different vacua of the SU(2) Yang-Mills theory and the corresponding inequivalent instanton field configurations are labeled by a number $n$, called the Pontryagin index.
How do the instantons evolve in time to change a vacuum labeled by $n=n_1$, for example, to another vacuum $n=n_2$?
Since instantons are also labeled by the integer $n$, which instanton is responsible for the above vacuum transition i.e., from $n=n_1$ to $n=n_2$?
