DPP has quite a bit of ability to edit JPG files; the basic window with exposure, picture style, etc. isn't useful nor is the digital lens optimizer tab but the remaining 6 tool tabs all work with jpg files. For example, you will need to make brightness and contrast settings in the tone curve adjustment tab.
I shoot everything in RAW which does allow a lot more versatility but all isn't lost with DPP if you have jpg files. Other programs like the Adobe creative cloud suite will also work with JPG but I bet that DPP will do everything you need to do if you are just needing to make some minor adjustments to your jpg images.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
