From The Document Foundation Wiki
- mark the cell range containing the data to convert
- press Ctrl+M or right click on the marked range to get the context menu and select Clear Direct Formatting
- if your locale does not use the separator(s) that are required for the text to be converted to number then temporarily apply a number format of a locale that uses the desired separator(s), for example English-US to get the . dot decimal separator; you can then later after the conversion apply the desired final number format of your locale. Either that, or replace the decimal separator, skip to the next example further below.
- press Ctrl+H or choose Edit -> Find & Replace from the menu
- enter Find: .+
- enter Replace: $0
- under Other options activate
- Current selection only
- Regular expressions
- hit Replace All