What should I do if the upgrade advisor app is not available in my phone?
How can I update my phone into W10?
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Your phone already has Windows 10 installed. If you don't get the Creator's Update it might be because your phone is not supported: https://www.theverge.com/2017/4/6/15203922/microsoft-windows-10-creators-update-mobile-update-list
You can join any Insider Preview Channel and hope to get the latest version from there: https://www.cnet.com/how-to/windows-10-creators-update-download-install/ ("Join the Windows Insider Program")
But be aware that Insider builds may have issues and you won't get proper support. If a build messes up your phone you'll need to restore it: https://support.microsoft.com/en-us/help/12379/windows-10-mobile-device-recovery-tool-faq
What device do you have and what exactly are you trying to do?