You have delivered a product to a client on time, on budget, and to specification, however the client is not happy with the result. What is the next thing that should happen?

It depends on what the product is. If it is a once off small item for private use it may be fixable easily or may not need to be changed. If it is a large order of products from a regular bulk contractor then you need to discuss the matter because you do not want to lose a valuable large customer. If there is room for interpretation within the order and you may have got it wrong then you need to consider that also. If appropriate negotiate with the customer and alter the contract if necessary. Discuss with the client what went wrong and how it can be prevented in the future. 

Thanks  for invitation.., I agree with Mr. Mohamed Farooq answer

thanks for invition ,,,,,,,,,, i agree with experts answers

I think you have to change the product 

customer satisfaction is the main aim, first we should be sure that client has tested the deliverable correctly and he is not satisfied then we should apply some change requests to meet the client's needs.

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My opinion

You're done. PMI assumes that as the project manager, you have been obtaining customer sign-off

for every intermediate deliverable on the project and that the customer has accepted those deliverables.

If there were issues with the deliverables, you would have found this out long before the end of the

project. The contract has been fulfilled according to scope, the budget, and the timeline. Closeout

contract and submit your final bill