After light excites electrons and they leave photosystem ii to travel down the first electron transport chain. how are these electrons replaced to photosystem ii?

Through the photolysis of water molecules

Explanation:

When the electrons in photosystem II gets excited, released and travel down to the electron transport chain, the electrons are replaced by the electron released by the cleavage/photolysis of water.

Water molecule is cleaved resulting in the formation of hydrogen ion, oxygen gas and electrons. The electrons then replace those lost by photosystem II.