You would have to go to your database and export your field as a CSV file. Then open file with excel.
Note: If you're using php my admin for your export, do not use
CSV for MS Excel. Use the CSV option. AsCSV for MS Exceldisplays the values in a comma format rather than columns.
In Drupal delete the field and create a new decimal field. Go to a node and enter a value for your decimal field.
Go back to your database and find the decimal field, export as CSV. Then open on excel.
Use the values from your 1st excel to copy the node id's and the field value and paste them onto your 2nd excel and save.
Import 2nd excel to your decimal field and check-mark replace table data with file.
Clear/flush the cache to see changes.