if you mean the Project Lead of a given project - project settings > details and edit the Project Lead at the bottom of the page. Deactivate old user if desired.
if you mean you want to change the Jira admin - add the new user to the admin role (jira settings > user management) and deactivate (don't delete) the user that departed. Note if there are automations, etc. that the previous admin had attached to their user you should modify these before deactivating them.
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