You can tell the shopping cart database the next order number to use when placing future orders. The database engine keeps track of the highest number ever used and will only use that number from then on. This means you can increase the next order number, but not decrease it. The cart uses this number when the next order in the store is placed.
Here is how to set it.
1. Under the Configuration menu area, choose Store Settings and then General Settings from the options shown there.
Once on the screen that opens, click the Options tab, and then Checkout.
Here you can set the next order ID. You only need to do this once, the cart will start counting from there then on. The setting screen shows the actual value that will be used for the next order on the right hand side - the setting you have previously set might be quite lower than that which is normal because you only need to set it once.
Once you save this setting, the next order you receive after the message should use the new number. 
Remember the number can only go up, not down, so don't choose a number that's too high to begin with unless you plan on using it.
