If the format order is identical to your native date format, then you could use the standard text replace function (Ctrl+H), replace the
- and Excel will re-parse them automatically.
If it is not, then you can use DATEVALUE and other functions to extract the date components separately and combine them in the proper format.
Example: if xx.xx.yy is actually mm.dd.yy, then you can do:
This uses the YYYY-MM-DD format, which is platform- and region-independent. You'll get the raw (int) value though, so you'll need to choose the cell format manually, but after that it'll work.