1. Simple search
In green horizontal navigation bar on the Project Home page, click on "Find issues". The simple search is the default search option on Backlog. With the simple search, you can filter issues by issue status, category, milestones. If you want to narrow your search further, click on "Advanced Search"
2. Advanced search
In advanced search you will be able to specify multiple search conditions. Here, you will also be able to search using keywords found in the issue.