how to make numeric field not accept negative value

CHECK Constraint[^]

EDIT
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Before update, check the calculation produces the negative value or not. If negative, raise exception else do update. Sample below.
Here we're checking the StockQty before update

SET @StockQty = SELECT StockQty FROM TblStock WHERE ItemID = 'SomeItemID';

IF @StockQty < @RequiredQty -- @RequiredQty value from your code as parameter to stored procedure
BEGIN
     PRINT 'Stock Qty is less than Required Qty' --Add exception block here based on your requirement
END
ELSE
BEGIN
     UPDATE TblStock SET StockQty=StockQty-@RequiredQty WHERE ItemID = 'SomeItemID';
END 

By the above same logic, you could do this in front end.