To change the top margin for just one page in Microsoft Excel, you can follow these steps:

1. Click on the page where you want to change the margin.
2. Go to the Page Layout tab in the ribbon.
3. Click on the Margins drop-down menu.
4. Select Custom Margins at the bottom of the menu.
5. In the Page Setup dialog box, click on the Margins tab.
6. Under the Margins tab, you will see a section called "Pages."
7. In the "Pages" section, select the "Apply to" drop-down menu and choose "Selected text" or "This point forward," depending on your needs.
8. In the "Margins" section, adjust the top margin to your desired size (e.g. 0.5 inches).
9. Click OK to apply the changes.

This should change the top margin for the selected page(s) only, without affecting the rest of the document. If you need to change the margins for multiple pages, you can select them all before following these steps.