To change local time on VDO digital tacho you need to follow this steps:
Press OK to bring up the menu
use arrow down to find local time setting
you will only be able to change time in 30 min blocks
Press ok then use arrow down to find settings and from settings select local time.
If your tacho clock is out and you need to change UTC (universal time coordinated) you need to follow the same steps but you will have to select UTC time set not local time.
You can change UTC time by minutes, however that change is limited to 20 min. If you need to adjust the time to more than that your unit will have to be re-calibrated
Your drivers hours are monitored in UTC time. UTC time is same as GMT so in UK winter time it will be the same as local time and in the summer it will be 1 hour out.