OP:PRINT OUT this reply and check it off step by step.
First, go to this page (Mr. Macintosh):
Download macOS Sequoia full installer with one click straight from Apple! Every version of Sequoia Beta & Final available all in one place!!

mrmacintosh.com
This is a page that lists ALL released Seqoiua installer files.
I believe the one you want is this one (installs version 15.7.2):
You can just click this link above to download it.I recommend that you download this to your desktop, or MOVE IT to your desktop from the downloads folder.
It will be a small "pkg" file.
Next
Double-click this pkg file to open it.
It will offer to "install" the Sequoia app.
DO THIS.
It will download a BIG file from Apple's servers.
HOWEVER...
This DOES NOT install the Sequoia OS directly.
Instead,
it installs the Sequoia install app into your applications folder.When done,
go to your applications folder and launch the Sequoia installer.
Start clicking through.
The Mac will reboot several times, and the screen will "go dark" for a minute or more with no other indication of activity.
Just be patient.
When done, you should see the welcome screen.
Do any setup you need to do.
But if you get this far, you're now upgraded to Sequoia.
Good luck.