To change the number of decimals for a specific metric you will want to start in the "Metrics" section. From here, you will be directed to a list of the metrics you have in Visible. Click on the metric name you would like to edit.
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.