How do I batch change the date taken information in EXIF data?

jhead is capable of doing this.

Let's say you know a certain picture was taken on 2017-04-19 16:20 but the current date is showing as 2007-09-01 00:15, you can adjust all jpg pictures in a folder to the correct time by doing:

jhead -da2017:04:19/16:20-2007:09:01/00:15 *.jpg

Here is an extract from the manual:

DATE / TIME MANIPULATION:
   -ft        Set file modification time to Exif time
   -dsft      Set Exif time to file modification time
   -n[format-string]
             Rename files according to date.  Uses exif date if present, file
             date otherwise.  If the optional format-string is not supplied,
             the format is mmdd-hhmmss.  If a format-string is given, it is
             is passed to the 'strftime' function for formatting
             In addition to strftime format codes:
             '%f' as part of the string will include the original file name
             '%i' will include a sequence number, starting from 1. You can
             You can specify '%03i' for example to get leading zeros.
             This feature is useful for ordering files from multiple digicams to
             sequence of taking.  Only renames files whose names are mostly
             numerical (as assigned by digicam)
             The '.jpg' is automatically added to the end of the name.  If the
             destination name already exists, a letter or digit is added to
             the end of the name to make it unique.
  -nf[format-string]
             Same as -n, but rename regardless of original name
  -a         (Windows only) Rename files with same name but different extension
             Use together with -n to rename .AVI files from exif in .THM files
             for example
  -ta<+|->h[:mm[:ss]]
             Adjust time by h:mm backwards or forwards.  Useful when having
             taken pictures with the wrong time set on the camera, such as when
             traveling across time zones or DST changes. Dates can be adjusted
             by offsetting by 24 hours or more.  For large date adjustments,
             use the -da option
  -da<date>-<date>
             Adjust date by large amounts.  This is used to fix photos from
             cameras where the date got set back to the default camera date
             by accident or battery removal.
             To deal with different months and years having different numbers of
             days, a simple date-month-year offset would result in unexpected
             results.  Instead, the difference is specified as desired date
             minus original date.  Date is specified as yyyy:mm:dd or as date
             and time in the format yyyy:mm:dd/hh:mm:ss
  -ts<time>  Set the Exif internal time to <time>.  <time> is in the format
             yyyy:mm:dd-hh:mm:ss
  -ds<date>  Set the Exif internal date.  <date> is in the format YYYY:MM:DD
             or YYYY:MM or YYYY

An even more powerful option is ExifTool.

Picasa has this built-in. Tools => Adjust Date and Time....

exiv2 is a command line tool to manipulate exif data. Supported image formats are JPEG, Canon CRW and Canon THM. PNG is read-only.

If you want to set the file date to the exif date you can use exiv2 with the following option.

-t Set the file timestamp according to the Exif create timestamp in addition to renaming the file (overrides -k). This option is only used with the ’rename’ action.