You could use a helper column and enter the formula:

=VALUE(A1)

and copy this down. You could fix the values using <copy, Edit | Paste
Special | Values | OK, and then paste the values to overwrite the
originals.

Another way is to enter a 0 into a blank cell, then <copy that cell,
then highlight all the cells that you want to convert and Edit | Paste
Special | Add | OK then <Esc.

Yet another way is to highlight the column(s) with numbers in, then
Data | Text-To-Columns and on the first panel click <Finish

Hope this helps.

Pete