To change the number of decimals for a specific metric you will want to start in the "Data" section. From here, you will be directed to a list of the metrics you have in Visible. You will need to navigate to the metric you would like to edit - and click the "Settings" icon on the ride side as shown below:
After you click "Settings" the options and format for your metric will appear as shown below:
Next, make any necessary changes to the format and rounding, click "Save", and your new format will be reflected across all charts and dashboards.