How do I change my iCloud email password?

Title and body of the post do not match. If you do not remember the password then you won't be able change it.

iCloud email password is same as your Apple ID password --> Click->If you forgot your Apple ID password - Apple Support

See Set up and use iCloud for Windows https://support.apple.com/en-us/HT201391. My read, from this document, is that you should use:

See If you forgot your Apple ID password https://support.apple.com/en-us/HT201487

And you undoubtedly will need to see If you forgot the answers to your Apple ID security questions https://support.apple.com/en-us/HT201485

Before trying to answer your security questions is very important that you think about your questions and their answers carefully and try to recall all details you can. If you don't answer them correctly enough, then you easily can lose control of your account.

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If you recovery your password, use:

Change your Apple ID password https://support.apple.com/en-us/HT201355

Here is What to do after you change your Apple ID or password https://support.apple.com/en-us/HT204071

Since you are using a non-Apple email program, you need to enable 2 Factor Authentication on your Apple ID and create an app specific password to use. If your account won’t allow 2 Factor Authentication, try 2 step verification.

Apple ID -Two-factor authentication          

 Two Factor Authentification availability.     

 Apple ID - Using app-specific passwords.     

Apple ID - Two Step Verifications FAQ