In Drive, select the document you would like to work on and click "Edit" in the toolbar. Your document will open and you can start editing as normal.
Tips:
1. You can also edit a document received in an email by clicking "Edit as new" next to the attachment.
2. Within Text, Spreadsheet or Presentation your recently changed documents will be listed on the left and templates are located in the middle. Click on any of these to start editing.