If you are looking to change the number format from a decimal (0.01) to a comma (0,01) Excel typically uses the system defined decimal separator. Depending on which operating system you are using, will determine how to change that setting. Somewhere in the Control Panel -> Language/Region settings you can change the decimal separator.
If you are using Excel 2010 (I don’t believe earlier versions have this), you can click File -> Options -> Advanced, then towards the bottom of Editing Options, you can un-check the box that says Use System Separators. Change the decimal/thousands punctuation to whatever you want (in this case the decimal separator to a comma, and thousands separator to a comma).
To change from a text string to a value (i.e. to preform a calculation), simply change the cell formatting from a text format to either Number, Accounting, Currency, etc. If, for some reason there is an extra € symbol in the cell afterwards (there shouldn’t be), you can do a find/replace on the double €€ and replace with only one.