Like all add-ons in Atlassian Marketplace, Issue History is set in two ways:
from Atlassian Marketplace;
from Jira admin page.
From Atlassian Marketplace
If you want to install the add-on from Atlassian Marketplace, please follow the steps listed below:
1. Type "Issue History saasjet" in "Search the Marketplace" field.
2. Find the add-on "Issue History" by SaaSJet at the search results.
3. Press the "Try it free" or "Buy now" button.
4. Select your site on the atlassian.net to install the add-on, then click "Start free trial"
5. You're all set!
From Jira admin page
If you want to install the add-on from Jira admin page, please follow the instructions below. (You need to be a JIRA Administrator in order to install Issue History)
1. First, click on "Jira Settings" and then choose "Apps".
2. Go there to "Find new add-ons" page.
3. Type "Issue History saasjet" in the search box and then click on it.
4. Click "Free trial" or "Subscribe now" to download and install the app.
5. You're all set!