An Audit's Planned Date can only be modified when changing its status from Draft to Planned.
However, if you're trying to update it but the Audit status does not change nor is the Planned Date modified, this may have to do with the toggle Sign off by all Hosts required in Audits > Module Settings > Signature Settings.
If Sign off by all Hosts required: OFF
If the Audit whose Planned Date you want to change has multiple Hosts (1) and Sign off by all Hosts: OFF (2), upon changing the Planned Date (3), the Audit's status will automatically be pushed to Planned after only one of the Hosts has signed off (4) and the Planned Date will be updated (5), while the original will be stricken through (6):
If Sign off by all Hosts: ON
If the Audit whose Planned Date you want to change has multiple Hosts (1), and Sign off by all Hosts: ON (2), upon changing the Planned Date (3), the Audit's status will be remain Draft (3).
Only after the last Host has signed off (4) will the Audit's status be pushed to Planned (5) and the Planned Date updated (6), while the original will be stricken through (7):