I assume that you exported the cashbook journal from the incorrect cashbook? Did you take note of which folder you exported it to? If you did not take note, do the export again and this time take note.

If you know which folder it was exported to, and you named the export file without the .csv file extension, then the file will be a simple text file which you can open with Notepad.

Open the file with Notepad - each line will have a reference to the cashbook account code, towards the right side. For example 8400000. Use Find and Replace to change that code to the correct cashbook code (don't insert a slash in the code, i.e. use 8400000 not 8400/000).

Save the edited file, and then import it into the correct cashbook.