You have the following tiled dashboard that has one sheet.

In Tableau, to replace a sheet on a dashboard, you can simply drag the desired sheet (Sheet2 in this case) from the sheets list onto the dashboard area where the current sheet (Sheet3) resides. This action will replace the existing sheet with the new one. Right-clicking and adding to the dashboard adds an additional sheet rather than replacing, and the context menu options mentioned in the other choices are not the standard methods for replacing sheets within a dashboard.