Use the formula `=UPPER(A1)` in a new cell
Press Enter
Copy the formula down for other cells if needed
Copy the results
Paste Special as Values to replace the original text
Use Flash Fill if available by typing the uppercase version and pressing Ctrl+E
Use Power Query with the Text.Upper transform for larger datasets
Use a VBA macro to convert selected cells to uppercase
Use the Change Case feature if available in your Excel version or add-in