If your Jira Admin granted you rights to use the Switcher for Jira app, you can find the relevant section in your profile menu and can behalf of another user. Navigate to the Switcher settings, select a user and push Login – now you are able to perform the actions in the system on behalf of the user you selected. You won't be able to find any user in the list of options – only those who have been included in the list of users available to turn into (specified within the Switcher Settings).
You may even transform into a user without Jira access. In this case, you'll see the Jira as if he/she would have access to it, you won't be able to create an issue though.
Note that all your actions on behalf of another user are logged. The audit log is available for your Jira Admin.
Another way to turn into a user is navigating to their user pic and selecting the Switch to user option in the More drop-down menu:
Don't forget to switch back when finished – just navigate to the profile picture and select the Switch back to my account option in the drop-down menu. In case you log out of Jira, you will be switched back to your account automatically.