How to Import a BACPAC file to Azure SQL and Overwrite Existing Database?
You are correct.  In Azure you cannot restore on an existing database. 


You have to restore with a different name.
Delete old database
Rename new one to old database name.


You have few ways to restore from .BACPAC file.


You can do it directly from your on prem .BACPAC location by using
.\sqlpackage.exe command line tool.

.\sqlpackage.exe /a:Import /sf:C:\filename.bacpac /tsn:ServerName.database.windows.net /tdn:destinationDBName `
 /tu:adminaccountName@serverName /tp:$credentialPW
You can also use the copy that you uploaded in your storage account.

$ResourceGroupName = "RGName"
$ServerName = 'ServerName'
$DatabaseName = "DestinationDBName"

$StorageName = "StorageAccountName"
$StorageKeyType = "StorageAccessKey"
$StorageUri = "http://$StorageName.blob.core.windows.net/swwstoragecontainer/BackpacFileName.bacpac"
$StorageKey = "***********************************" 

$credential = Get-Credential

$importRequest = New-AzureRmSqlDatabaseImport -ResourceGroupName $ResourceGroupName -ServerName $ServerName 
 -DatabaseName $DatabaseName -StorageKeytype $StorageKeyType -StorageKey $StorageKey
 -StorageUri $StorageUri -AdministratorLogin $credential.UserName -AdministratorLoginPassword $credential.Password `
 -Edition Standard -ServiceObjectiveName S0 -DatabaseMaxSizeBytes 50000

Get-AzureRmSqlDatabaseImportExportStatus -OperationStatusLink $importRequest.OperationStatusLink
In portal you can directly import the .BACPAC file into your server as a database.