Sage 50 is different from some other software that has a 'cost' field for items that have never been purchased / adjusted into inventory / entered as setup beginning inventory values.

If your inventory ledger is in agreement with your General Ledger inventory balance, and it's out of 'history' mode, then you can enter inventory adjustments to correct your inventory balances.

In the Inventory Adjustment screen there are three columns, the Quantity, Per Item, and Amount.
If you need to correct the cost of, say 4 items that should be $10 each, but they're now in at $7.00 each you can either:
Adjust out 4 at the $7 each cost that's on your books, and put 4 back in at $10 each,

or if you're using Average cost:
Adjust an additional ($3*4) $12 to that item's book value.

Or...

Do nothing, and as inventory is sold, and replaced, the cost will be corrected.

(i.e. if you sell 2 at a cost of $7, $14 cost goes out, then when 2 @ $10 is bought to replace it, $20 goes back in, so instead of being out by $3.00 as above, it's on average $1.50 too low, per item).

I hope that helps, please post back.