Below are the steps to do an Index Reset thru GUI
1. Open Central Administration > Manage Service Application.
2. Click on Search Service Application.
3. On Search Administration Page on left side under Crawling you can see Index reset option.
4. Click on it, it will again ask for confirmation. Click on Reset Now.
You are done with index reset and now you can start full crawl on your content sources.
If you want Powershell to do an index reset. Run below command.
Parameters true is to disable alerts and ignore timeout error.
Happy SharePointing :)