Navigate to Business Central Admin Center through Partner Center
Log into Partner Center (https://partner.microsoft.com/en-us/dashboard/home)
Select Customers, find customer in list (or using search) and select customer
Select Dynamics 365 Business Central
You will now be in Business Central Admin Center.
If you are unable to access BC Admin Center...
you will need to login directly to the tenant or access the Partner Center through a different account that has access.
Navigate to https://businesscentral.dynamics.com/ and login.
Click on the gear icon in the top right and select Admin Center.
Modify Scheduled Update Date
Notice the list of environments, the Current Version, Next Update, and Scheduled Update Date
Open each environment by clicking on the blue link.
From this page, find the Modify link in the Next Update section
Click Modify to open a pane on the right where you can set the date
Click on the date to modify and select the preferred date. Click Schedule Update