About this passageThe second verse addresses the reader who cannot take the first one — who does want the hundred years, and the house, and the work. It does not withdraw the offer. It gives a second route: keep acting, but perform the action for its own sake rather than for what it will bring you, and the results have no hold on you. Karma here means the deed and its consequence together, which is why the verse can say the consequence will not stick.
If one should desire to live in this world a hundred years, one should live performing Karma (righteous deeds). Thus thou mayest live; there is no other way. By doing this, Karma (the fruits of thy actions) will not defile thee.
In plainer words
Suppose you do want to live your hundred years. Then live them doing what is right, and keep doing it. There is no other way through. Do it like that and what your actions bring back will not stain you.
A restatement in modern English, written for this collection. The passage above is the text; this is one way of putting it.