Insert Data else Update FROM a CSV file?

I am currently using an SQL job to import the contents of a CSV file (BULK INSERT). I use a LEFT JOIN statement to check if there are duplicates and to not insert them if any are found. However, I would like to instead UPDATE a column if their are duplicates.
Here are the fileds of my CSV file:
|UserType|Lname|Fname|Mname|ContactNo|Email|Password|Balance|Status

Here is my LEFT JOIN statement.

INSERT INTO tblAccount
SELECT CSVTable.AccountID,
CSVTable.UserType,
CSVTable.Lname,
CSVTable.Fname,
CSVTable.Mname,
CSVTable.ContactNo,
CSVTable.Email,
CSVTable.Password,
CSVTable.EPurseBalance,
CSVTable.AccountStatus
FROM CSVTable
       LEFT JOIN tblAccount 
         ON CSVTable.AccountID = tblAccount.AccountID
WHERE tblAccount.AccountID  IS NULL

I would like to UPDATE existing records if duplicates are found. Let's say a user's AccountStatus. If a user is already on the Accounts table BUT his account was deactivated, I would like to update not the entire row but only the AccountStatus column.

My current statement discards duplicate entries regardless if some columns are different. How can I insert if data is non existent but update a column if its already existing?

Any efforts to answer my question will be greatly appreciated!

EDIT I tried the MERGE command. Here is my statement:

MERGE 
   tblAccount AS target
USING 
   CSVTable AS source
ON 
   target.AccountID = source.AccountID 
WHEN MATCHED THEN 
INSERT INTO tblAccount
SELECT CSVTable.AccountID,
        CSVTable.UserType,
        CSVTable.Lname,
        CSVTable.Fname,
        CSVTable.Mname,
        CSVTable.Address,
        CSVTable.Gender,
        CSVTable.Birthdate,
        CSVTable.ContactNo,
        CSVTable.Email,
        CSVTable.Password,
        CSVTable.EPurseBalance,
        CSVTable.AccountStatus
FROM CSVTable
       LEFT JOIN tblAccount 
         ON CSVTable.AccountID = tblAccount.AccountID
WHERE tblAccount.AccountID  IS NULL



WHEN NOT MATCHED THEN 
   UPDATE SET tblAccount.AccountStatus = CSVTable.AccountStatus
; 

This doesn't work and SQL server throws me errors:

Msg 156, Level 15, State 1, Line 8
Incorrect syntax near the keyword 'INTO'.
Msg 156, Level 15, State 1, Line 29
Incorrect syntax near the keyword 'WHEN'.

I tried inserting my LEFT JOIN statement in my MERGE but to no avail. Any ideas? Thanks!