JPEG and JPG files are of the same format, so you can simply rename the extension should you wish or need it. There is no need to look for some kind of converter, and even if you used one, it will just rename the files anyway. This is usually the case with some of the online converters.
Suggested software and links: jpeg to jpg converters
A 2D Design is drawing and design tool
A bitmap graphics editing program
Photo editing software, an alternative to Adobe Photoshop
Graphic editor for diagrams and simple graphics from OpenOffice.org suite
A stylish painting package
A picture conversion program
A photo editing software distributed as a part of CorelDRAW
A set of applications used by professional graphic designers. Available for Windows and MacOS.
A digital photo editing and converting software for Linux
A simple vector graphics editor
A program for creating albums, calendars or stickers from digital photos
Photo enhancing program for Mac
Photo editing tool for Mac
A free graphics and photo editor and authoring tool
GNU Image Manipulation Program
Mac version of the popular graphics editor
An image manager, editor and converter for Mac
A program for editing raster technical images
An image viewer for Linux KDE desktop
A fast and inexpensive app for creating impressive landscape images
A photo editor for Mac OS X
Picture image editing tool for Mac
A graphics editing software for ´Mac
Graphic and diagram software from LibreOffice suite
A powerful 2D design software with 25 year history
A vector-based drawing software
A popular painting program
Raster graphics and 3D computer graphics application, which is a refresh of Microsoft Paint.
A free photo and image editor
A complete image retouching program
A video & picture collage maker
An online graphics converter and resizer
A 3D CAD translation software
A basic Apple MacOS document and image viewer
A dedicated conversion software
Sketching and painting software
Digital drawing app
A 3D general-purpose modeling program
3D modeling application for Mac
A logo design editor
A sprite sheets making software
A picture editing software for Mac
A painting program for Windows
A simple yet graphics editor for Mac
Desktop publishing software
A popular utility for viewing and converting graphic files