Files with .2DA extensions are two-dimensional arrays, or tables, or spreadsheets.
The Witcher did not introduce any change here - they are the exact same format as they were in NWN or KOTOR. This means that you can use any of the tools already available.
The 2DA files are plain text, and the columns are delimited by whitespace characters (e.g. space and tab) - with small 2DA tables a simple text editor will do. For larger tables however, I recommend some more serious tool - for example, Excel.
one Excel method Edit
A piece of advice to Excel users: pay attention for the delimiter characters when you import the 2DA file into Excel the first time. Excel will detect either space or tab as a delimiter, but never both - so you will need to add the other as a delimiter to the import settings; while there, be sure to check the "treat consecutive delimiters as one" option, too. When you have saved your 2DA file the first time, remember that the character in the header (2DA V2.0) MUST be a space! If it's a tab, use notepad or something to change that back to space! After you have done this import/save hassle once, you don't need to do this ever again (well, not for the file you have updated, anyway).
a simpler Excel method Edit
Simply do not save any Excel-specific formatting, use "save as..." and leave things as Excel "sees" them, i.e. if it thinks it was a text file, then fine.. whatever. When the pop-up asking you if you are sure you want to discard any Excel formatting is presented, just don't keep any. You will be asked again when closing the file, just don't and it works just fine.