How do I correc an error of a stock symbol?

Having a good backup is always a good idea when making such changes.

First check that you do not have two securities representing the same stock.  Ctrl-Y to pull up the security list.  Click the ticker symbol header to sort the list by ticker.  Make sure a security does not currently exist with the 'correct' ticker.  

As long as the ticker does not exist in your file, you can Edit the security details for the applicable security and type in the correct ticker.  Quicken will then prompt you:

Do you want to copy the old prices to the new ticker?  Answer accordingly, usually yes, but maybe not in this case.

Do you want to delete the prices associated with the old ticker?  Answer accordingly, often yes, but not always.  

Once you have corrected the ticker you can download historical quotes for the corrected security.  That process will bring in daily prices for the last 30 days, end-of-week prices back to a one-year ago time, and end-of-month prices from then back five years.  You can choose the time period you need.  There are some additional tweaks you can make if needed