1. Select the cell or range of cells that contain the large numbers you want to abbreviate.
2. Right-click on the selection and choose "Format Cells" from the context menu.
3. In the Format Cells dialog box, go to the "Custom" category.
4. In the "Type" field, enter the following custom number format: 0.0,," Bn"
5. Click "OK" to apply the format to the selected cells.

This custom number format will display the numbers in billions with one decimal place and the "Bn" abbreviation. For example, 2,500,000,000 will be displayed as 2.5 Bn.

Note that the comma after the first zero in the format separates the thousands, while the double comma after the second zero separates the millions. The "Bn" text at the end of the format will be displayed as is.