I entered the wrong amount in the "On Hand" amount field in Inventory Item

Create an inventory adjustment.

If you added the "on hand" value when you added the item, and you made an error, another approach (Jeanne's approach is a correct one, I'll note, but not the only one) is to make a correction to the original entry. Right click on the item in the item list and select the "QuickReport". Set the date range to "all". You will see an inventory adjustment for that first value that  you entered - double click on it. That will open the entry, which is an inventory adjustment. You can correct the quantity there. Don't do this if the original entry was in a closed accounting period.