Hi Adreas,
Do you mean that "Manage" option is not available in your power platform admin center?

If so, please check whether your account has system administrator role:
For your second question, the availability of September 2020 update varies by region, we couldn't select it manually.
In other word, we need to wait for the update packages to be pushed to us.
But since you have enabled 2020 wave 2 early access, then the update packages shall be available.
I have enabled the early access to my environment, and 1.35.10065.2064 is available for me.

Try to fix "Manage" option issue at first to be able to see marketing application update page, then check whether September update packages would be available for you.
Also, try whether you could open legacy applications center directly via the link.(X is your Xrm region number.)
port.crmX.dynamics.com/.../Index.aspx
Regards,
Clofly