There can be two scenarios in which multiple issues are updated using the Update Issues post function. One, when you know which issues to update and the other when the conditions or JQL provided fetch multiple issues as mentioned in Use JQL to find issues to update.
In the former scenario from the preceding section:
Select the issues to be updated from the Issues to Update field in the Detailed Fields tab as the following, and provide the other values as required to update the selected issues accordingly.: