Most states have a simple "Amendment to the Contract" form, which can be filled in to state that the contract is amended to reflect a sale price of ............... , and have the seller(s) sign it. Don't forget to change your down payment/mortgage amount which will be influenced by the actual sale price!
Your inspection contigency in the original contract states how quickly you must inform the seller of your dissatisfaction with the inspection, whether you musrt provide the seller with a written report from the inspection, and whether the seller has the option of resolving those issues by making repairs/replacements, etc. rather than granting an allowance to you. Check with your agent, if you have one. Otherwise, reread that section of your purchase agreement, present the seller with your preference for a price reduction, and cancel the contract before any time limits have expired if you're not satisfied with the answer. You can always write a new contract if the situation changes.
Hope that helps.