Identify the transaction number of the charge you have posted to the wrong project. (For example, if it's a supplier invoice, you can identify this by looking on the supplier account).
Now go to the Corrections screen (File menu -> Maintenance -> Corrections) and click "Find" at the bottom. Enter the transaction number and click "Find first". The transaction in question will be highlighted after a moment or two.
Double click on it to open the dialogue for modifying some aspects of the transaction. Near the bottom should be a breakdown of the transaction (if it affects more than one nominal code - if it doesn't there will be one line, highlighted). Make sure the right part is highlighted, then click "Edit".
Somewhere in the next screen you should be able to change the project ref and the cost code. Once you've done so, click "Close" in this dialogue, and then "Save" in the previous one.
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Vince M Hudd - Soft Rock Software
(I only came here looking for fellow apiarists...)