To manually convert Zimbra to Office 365, you can follow these steps:
Export your Zimbra mailboxes to PST files. You can do this using the Zimbra Migration Tool or a third-party migration tool.
Import the PST files into Office 365. You can do this using the Outlook Import/Export Wizard.
Change your email settings to point to Office 365. This will involve updating your email client settings, such as your email address, password, and server settings.

Here are the detailed steps on how to do each of these tasks:
Exporting your Zimbra mailboxes to PST files:
Log in to your Zimbra account.
Click on the Mail tab.
Click on the name of the mailbox that you want to export.
Click on the Settings tab.
Scroll down to the Export section.
Click on the Export button.
Select the PST file format and then click on the Next button.
Select the location where you want to save the PST file and then click on the Save button.

Importing the PST files into Office 365:
Open Outlook.
Go to File > Open & Export > Import/Export.
In the Import/Export Wizard, select Import from another program or file and then click Next.
Select Outlook Data File (.pst) and then click Next.
Click on the Browse button and select the PST file that you exported from Zimbra.
Click on the Next button.
Select the mailbox where you want to import the emails and then click on the Finish button.

Changing your email settings to point to Office 365:
This will involve updating your email client settings, such as your email address, password, and server settings. The specific steps for doing this will vary depending on your email client.

Regards,
Peter