Press on the trigram left of the tabs. With the project properties dialog open, you can adjust decimals for percents or counts (if applicable).
Note: Decimals for percents/counts only determines decimals for frequencies. If you have a group that is compacted to "mean" use valueFormat.
In groups that are compacted to a statistic like "mean", adjusting decimals in project properties will not apply to these values.
valueFormat uses d3 format notation. "0f" will display numbers round to whole integers, "0.1f" will display 1 decimal point, "0.2f" displays 2 decimals, and so on.
Additionally, you can add a percentage sign to valueFormats. E.g. "0%" will display fractions like 0.24 as percentages e.g. "24%"