To update an existing scheduled task run the same command as above with any desired modifications. You will be prompted with, “WARNING: The task name “Reboot” already exists. Do you want to replace it (Y/N)?”

Schtasks supports the following options
SCHTASKS /Change [/S system [/U username [/P [password]]]] /TN taskname
     { [/RU runasuser] [/RP runaspassword] [/TR taskrun] [/ST starttime]
       [/RI interval] [ {/ET endtime | /DU duration} [/K] ]
       [/SD startdate] [/ED enddate] [/ENABLE | /DISABLE] [/IT] [/Z] }

Description:
    Changes the program to run, or user account and password used
    by a scheduled task.

Parameter List:
    /S       system      Specifies the remote system to connect to.

    /U       username    Specifies the user context under which schtasks.exe
                         should execute.

    /P       [password]  Specifies the password for the given user context.
                         Prompts for input if omitted.

    /TN      taskname    Specifies which scheduled task to change.

    /RU      username    Changes the user name (user context) under which the
                         scheduled task has to run. For the system account,
                         valid values are “”, “NT AUTHORITYSYSTEM” or “SYSTEM”.
                         For v2 tasks, “NT AUTHORITYLOCALSERVICE” and
                         “NT AUTHORITYNETWORKSERVICE” are also available as well
                         as the well known SIDs for all three.
    /RP      password    Specifies a new password for the existing user
                         context or the password for a new user account.
                         This password is ignored for the system account.

    /TR      taskrun     Specifies the new program that the
                         scheduled task will run.

    /ST      starttime   Specifies the start time to run the task. The time
                         format is HH:mm (24 hour time) for example, 14:30
                         for 2:30 PM.

    /RI      interval    Specifies the repetition interval in
                         minutes. Valid range: 1 – 599940 minutes.

    /ET      endtime     Specifies the end time to run the task. The time
                         format is HH:mm (24 hour time) for example, 14:50
                         for 2:50 PM.

    /DU      duration    Specifies the duration to run the task. The time
                         format is HH:mm. This is not applicable with /ET.

    /K                   Terminates the task at the endtime or duration time.

    /SD      startdate   Specifies the first date on which the task runs.
                         The format is mm/dd/yyyy.

    /ED      enddate     Specifies the last date when the task should run.
                         The format is mm/dd/yyyy.

    /IT                  Enables the task to run interactively only if the
                         /RU user is currently logged on at the time the job
                         runs. This task runs only if the user is logged in.

    /RL      level       Sets the Run Level for the job. Valid values are
                         LIMITED and HIGHEST. The default is to not change it.

    /ENABLE              Enables the scheduled task.

    /DISABLE             Disables the scheduled task.

    /Z                   Marks the task for deletion after its final run.

    /DELAY   delaytime   Specifies the wait time to delay the running of the
                         task after the trigger is fired.  The time format is
                         mmmm:ss.  This option is only valid for schedule types
                         ONSTART, ONLOGON, ONEVENT.

    /?                   Displays this help message.

Schtasks Examples:
    SCHTASKS /Change /RP password /TN "Backup and Restore"
    SCHTASKS /Change /TR restore.exe /TN "Start Restore"
    SCHTASKS /Change /S system /U user /P password /RU newuser /TN "Start Backup" /IT