Atlas CRM is available via the Atlassian marketplace. You can follow the instructions for installing apps from the marketplace in the Atlassian documentation.
You can find the app listing on the Atlassian marketplace.
Before installing Atlas CRM, make sure that Jira has enough memory. We recommend raising the amount of memory with at least 256m. Jira by default is started with -Xmx768m, in that case, raise it to -Xmx1g. More detailed information on resource usage is available in the Data Center compatibility section.
We recommend re-indexing Jira after installing Atlas CRM to make sure that Atlas CRM JQL functions work correctly. Fore more information about re-indexing Jira, visit the Atlassian documentation on search indexing.