You can use the QBE window to build an update query.
UPDATE YourTable SET YourTable.YourPrice = [YourPrice] - 15;
1. Open the QBE window.
2. Add the table
3. Select the column you want to update.
4. Change the query type to update.
5. In the UpdateTo cell place: [YourPrice] - 15
The square brackets are important. If you omit them, Access will interpret the whole thing as a string "YourPrice - 15" which won't work at all.