I have lightroom 4...just downloaded the 5.7.1
can't seem to buy serial number
If your operating system supports it, buy the upgrade to Lightroom 6 at 79 US Dollars, or whatever the price is in your country.
You might be able to find internet retailers that are still selling Lightroom 5.
Lightroom 5.7.1 is not sold as of now by Adobe, you may purchase Lightroom 6 if you would like from the following link : Products
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