In Cisco Packet Tracer, when you're in configuration mode, there isn't a specific "undo" command. However, you can achieve the same effect using several techniques depending on the type of command you're trying to reverse:

How to Undo in Configuration Mode in Cisco Packet Tracer

1. Use the Command History:

In Cisco CLI, you can access previously entered commands using the up arrow key to scroll through your command history. This can save time when redoing a command instead of typing it again.

Unfortunately, there's no automatic redo feature like in some other software, so you'll have to manually reapply the configuration. 

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2. Negating a command:

   - Precede the command with `no` to undo a specific setting. For example:

     interface fastethernet 0/1

     no shutdown

   To reverse this and shut the interface down again, use:

     interface fastethernet 0/1

     shutdown

3. Resetting configuration:

   - If you want to reset an interface or routing setting, use the `default` command to restore default settings. For example:

default interface fastethernet 0/1

4. Erasing the entire configuration:

   - To erase the entire running configuration, you can use:

     write erase

     reload

 This erases the current configuration and reloads the device.

Using these methods, you can effectively "undo" specific changes in Cisco Packet Tracer's configuration mode.