Frequency values
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.
Statistic values
In groups that are compacted to a statistic like "mean", adjusting decimals in project properties will not apply to these values.
Adjust decimals shown by setting valueFormat in "Edit properties".
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.
Result:
Additionally, you can add a percentage sign to valueFormats. E.g. "0%" will display fractions like 0.24 as percentages e.g. "24%"