The tool you need is called
jhead. Some OSs release it via their package managers, but this is the home of jhead, and you can download the latest version from there.
-ta Adjust time stored in the Exif header by h:mm forwards or backwards. Useful when having taken pictures with the wrong time set on the camera, such as after travelling across time zones, or when daylight savings time has changed. This option uses the time from the "DateTimeOriginal" (tag 0x9003) field, but sets all the time fields in the Exif header to the new value.
Adjust time one hour forward (you would use this after you forgot to set daylight savings time on the digicam)
jhead -ta+1:00 *.jpg
Adjust time back by 23 seconds (you would use this to get the timestamps from two digicams in sync after you found that they didn't quite align)
jhead -ta-0:00:23 *.jpg
Adjust time forward by 2 days and 1 hour (49 hours)
jhead -ta+49 *.jpg