The numbers in a report can be shown using a variety of formats:  currency, percent, with or without decimals.

To change the number of decimal places in a row, go to Reports (top bar)>Report name (left sidebar)>Edit (pencil at top right)>Edit Report.

Click on the row to be reformatted until it turns blue and reveals the edit options, as shown below, then use the Decimals option to select the number of decimals.

To set the number format for a new calculation (selecting currency, numeral, or percent), click on Outcome Format on the formula summary row (next to name of row) to reveal the selector as shown below.