Here's how you can lock a chart in Excel so that it won't update:

1. First, select the chart that you want to lock.
2. Next, right-click on the chart and select "Format Chart Area" from the drop-down menu.
3. In the Format Chart Area pane that appears on the right-hand side of the screen, click on the "Properties" tab.
4. Under the "Properties" tab, you should see an option called "Don't move or size with cells". Check the box next to this option.
5. Click on the "Close" button to close the Format Chart Area pane.
6. Finally, save your Excel file. Your chart should now be locked and will not update when you open the file.

By checking the "Don't move or size with cells" option, you are telling Excel to treat the chart as a separate object that is not linked to the data in your worksheet. This means that the chart will not update when you make changes to the data in your worksheet.