Procedure

At the upper right of the Jira screen, click the gear button and select [Manage apps] to display the Jira administration screen.

Open

From the left sidebar, click [Manage apps].

Open

Click [Upload app].

The modal for uploading add-on appears. Click [Browse] and select the plug-in (jar file) from your environment.

Make sure that the correct file has been selected and click [Upload].

The installation is now completed.

Reference

For details on the installation of the Jira Software plug-in, see Atlassian's official document Installing Marketplace apps as well.