The time display format within the app corresponds to the time display format that is currently set in Jira by an administrator.
A Jira admin can change the time display format by navigating to:
Jira administration > Issues > Time tracking
In cloud version, it will take about 15 minutes for the changes to take effect.
On the other hand, the time format for reports can be selected, without leaving the Reports mode. The reports can be generated either in current Jira time format, days, hours or minutes.