How did the data get ?How did the data get into your spreadsheet ?
For a quick one off fix. If the decimal is only appearing in the numbers and you don't have any text with the period in it then
• select any populated cell
• Ctrl+A (this will select all the data
• Ctrl+H will bring up the replace box
• Find What = the Period / Decimal
• Replace with = the comma
• Click on replace all