When editing, or altering, plugin files, you need to take care and make sure that you keep a backup of the original file, just in case you make any mistakes in coding. You can restore the original by uploading the backup that you kept, initially. You can do this by simply downloading the plugin files from your Web server via FTP before making any changes to the plugin files.
Additionally, WordPress provides you with a handy plugin editor feature. You can visit the Plugin Editor page in your Dashboard to edit plugin files:
The Plugin Editor page displays in your Web browser.
In the Select Plugin to Edit drop-down menu, select the plugin you want to edit.
The Edit Plugins page refreshes with the primary plugin file of the plugin you selected displayed in the text box on the left side of the page.
The right side of the Edit Plugins page displays a list of links to the files included within the plugin you selected in Step 2. The primary plugin file is always the one that is displayed, first, in the text edit box on the left (the primary plugin file is the one that contains the initial action hooks and filters).
Be sure to click the Update File button at the bottom of the Edit Plugins page in order to save and apply your changes.