I join the files that I download but WinRaR says "The file is corrupted." What will I do?

Well there is 2 things you can do.

First, open 1(one) rar file with WinRAR and click "Extract To". After that, Check “Keep broken files” then click "OK".

After it finishes extracting then you should have a complete uncorrupted file.

If your file is still corrupted then your Second Option is re-download the files. Maybe it is corrupted because you lose your Internet connection while downloading or you accidentally press "Cancel" upon downloading the file.