You would need to consult an (Indian) attorney as far as proper legal procedure is concerned, but the underlying legal principle is that you have a contract with someone (let us assume it is with HP), and there may be a breach of contract on their part. One step would be to ascertain that there is a breach, checking for clauses that allow them to substitute without notification. The "Replacement" item of the terms page suggests that they accept the obligation to replace in case of wrong shipment, however, if they previously informed you of the substitution and you accepted, then you are out of luck (see "order fullfilment"). They might be relieved of their ordinary obligations if you did not notify them of the defect in proper fashion, viz. the 14 day requirement; it is not clear who or how you are supposed to notify them.
Another is to be certain that the party refusing to perform is the party with whom you have an agreement: one can buy a name brand computer apparently from "Zompacked", but actually from a distributor, in which case you have to take up the matter with the actual seller, not the manufacturer (even if the manufacturer is willing to take partially satisfy the seller's obligations). This is relevant because the terms page states that "Service Provider shall be solely responsible for fulfillment of all orders", and HP is not the service provider, that other company is. You could check the language of the contract for a binding arbitration clause (requiring you to resolve disputes outside of the courts, most likely in a venue favorable to the company), though there is not such a clause in their terms. All that said, you also ascertain that the terms page that I located is applicable to your purchase. Online computer sales can involve multiple purchasing options, you you need to be sure the Terms that you agreed to were these.
Needless to say, non-legal recourse would be cheaper than trying to sue them.