First scene is talking about Malcolm, Siward and Macduff planning to go up the hill with branches with the army to Macbeth's castle. Second scene Siward walks in the castle and has a conversation with Macbeth until the next scene to where they fight till the death. And Macduff kills Siward because of the confidence the witches gave him for not being killed by someone who was born from a woman
Malcom, old siward and Macduff going up the hill
MACBETH meets old SIWARD
Macbeth kills Siward
Siward is dead
MACDUFF, old SIWARD and MALCOM move their army uphill with branches
Siward walks into Macbeth's castle and approaches Macbeth saying hes not afraid of him "Thou liest, abhorrèd tyrant. With my swordI’ll prove the lie thou speak’st."
"But swords I smile at, weapons laugh to scorn,Brandished by man that’s of a woman born." in other words he saying that his sword doesn't hurt him because he is a born from a woman