Trying to upgrade my macbook Air 13-inch, Early 2014 from OS X 10.9.5 (13F1911) to macOS Big Sur.

Michael6105 Said:

"Trying to upgrade my macbook Air 13-inch, Early 2014 from OS X 10.9.5 (13F1911) to macOS Big Sur.: [...]However the issue is that it keeps on saying that my Apple ID or Password is incorrect, which i know for certain it aint. Any advice please im losing my mind now."

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Upgrading the macOS:

Logging In with Your Account:

Is this a pre-owned Mac? If so, then take this back to the seller, and have them log out of iCloud on it. This is the link the seller should have used prior to selling it to you, including the iCloud logout: What to do Before you Sell, Give Away, Trade In, or Recycle your Mac - Apple Support. The seller should have known better.

Michael6105 Said:

"Trying to upgrade my macbook Air 13-inch, Early 2014 from OS X 10.9.5 (13F1911) to macOS Big Sur.: [...]However the issue is that it keeps on saying that my Apple ID or Password is incorrect, which i know for certain it aint. Any advice please im losing my mind now."

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Upgrading the macOS:

For future reference...

Updating the macOS:

Determining which Update(s) will Install:

I. Verify Compatibility:

A. Start Here: Learn About Compatibility and how to Identify your Specific Mac Model - Apple Support

B. How to Verify:

I Reveal your Model:

What is the Model of your Mac? To find out...

1. Go to: Apple menu
2. Click: About this Mac
3. View: Second Line-Down

then...

II. Compare the Results:

See if the Model you found is shown here:

• macOS Ventura is Compatible with these Computers - Apple Support
• macOS Monterey is Compatible with these Computers - Apple Support
• macOS Big Sur 11 Hardware Requirements - Apple Support
• macOS Catalina 10.15 - Hardware Requirements - Apple Support
• macOS Mojave 10.14 - Hardware Requirements - Apple Support
• macOS High Sierra 10.13 - Hardware Requirements - Apple Support
• macOS Sierra 10.12 - Hardware Requirements - Apple Support
• OS X El Capitan 10.11 - Hardware Requirements - Apple Support
• OS X Yosemite 10.10 - Hardware Requirements - Apple Support

III. If Compatible:

Back Up your Mac First:  Be certain you Create a Time Machine Backup of your Mac, so that way you can have something to restore your Mac from, should anything go wrong with the reconfiguration.

Michael6105 Said:

"Trying to upgrade my macbook Air 13-inch, Early 2014 from OS X 10.9.5 (13F1911) to macOS Big Sur.: [...]I already determined the compatibility, and it should work on my device”

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Upgrading the macOS:

Important to Know:

Back Up your Mac First:

Prior ro making any major change to your Mac, be certain you Create a Time Machine Backup of your Mac, so that way you can have something to restore your Mac from, should anything go wrong with the upgrade.

A. Is this Pre-Owned?:

So, is this a previously-owned device? If so, and you never had the original owner log themselves out of iCloud on it, then you may be out of luck. So, take this to the peering who is currently signed in to iCloud on it, and have them perform as instructed here, including the iCloud logout: What to do Before you Sell, Give Away, Trade in, or Recycle your Mac - Apple Support

B. Try Installing Using a New Administrator.

For temporary use, try creating a new Administrator user, seeing if you can install it through that user. Once installed, save what need be, and then log into you original user that is gong your this issue. Once logged in, trash the Temporary Administrator user you’ve just created. Go Here: Add a User or Group on Mac - Apple Support

You're getting this before the system downloads the installer, right? Once the system downloads the installer and starts running it, I think it would be asking you for your local Admin user name and password.

That means that the seller did not prepare the Mac properly before seling it - it is still associated with the seller's Apple ID. They would need to do that.