If you need to extend multiple lines, use the Extend Line geoprocessing tool. The tool extends lines to an intersection within a dangle length tolerance distance. The dangle length is the maximum length that a line will be extended; if no intersections are found within that distance, the line is not extended. The tool can extend all lines or just the selected ones.
This tool is useful for cleaning up features that were not digitized using snapping or were imported from another format, such as CAD. You might want to use the Extend Line tool in conjunction with the Trim Line tool, which removes lines shorter than the dangle length. In addition, after running these tools, it may be useful to build a geodatabase topology containing the Must Not Have Dangles rule and further refine the quality of your data.