(By THACH Lockevn, 1/4/2018 9:46:19 AM) To reset the search index, admin goes to IndexContent.aspx In CMS 11, IndexContent.aspx is included in the new EPiServer.Search.Cms package The URL has changed from /EPiServer/CMS/Admin/IndexContent.aspx to /EPiServer/EPiServer.Search.Cms/IndexContent.aspx
Source: Breaking changes (CMS 11)