Each Azure AD App Client secret will expire. Follow this guide to update it so that syncs will continue.
To update the client secret, navigate to the main settings page by selecting the cog in the top navigation bar.
Then, select "Manage" in the Azure AD Sync panel.
Then, select "Manage" next to App Credentials.
Copy Client ID
Azure Admin Portal
The next 1-3 steps will continue from the Azure admin portal
1. Locate the App registration that needs extra permissions
1-1 Click on App registrations
1-2 Enter copied Client ID
1-3 Click on found App registration
2. Select 'Certificates & secrets'
3. Create a new client secret
3-1 Select – '+ New client secret'
3-2 Expires = 2 years > Select 'add'
3-3 Select 'Add'
3-4 Key has now been generated in the Key field. Copy and store the key value.
4. Enter new secret in Archive AD Sync App Credentials
4-1 Enter new secret in the Key field
4-2 Select 'Update'