Check the box if you still have the package the microwave came in, because the manual might have gotten mixed up with all of the packing materials.
Go the microwave manufacturer's website. Search for your microwave model on the site and open the support page for it. Click the link to download the manual to your computer.
Call the manufacturer's customer service number. Explain to the representative that you need the manual for your microwave oven. There may be costs associated with mailing a replacement manual out to you.