On a digital tachograph all data is recorded in UTC time so nothing needs to be changed for that but if you want to change the local time that's displayed on the VU screen here's how.
- Changing "Local Time" on a Siemens Digital Tachograph
- Changing "Local Time" on a Stoneridge Digital Tachograph
- Changing the time on an Analogue Tachograph - Page 4
Thanks to Novadata for putting these video's on Youtube
Siemens Digital Tachograph
Stoneridge Digital Tachograph
Actia Digital Tachograph
Hope this helps.