You have two possibilities:
Via VitaDock®+ App:
Start the VitaDock®+ App and press on the Bluetooth icon at the upper right corner. If you already paired your device a gear symbol should appear next to your ViFit Connect Activity Tracker. If you press it you enter the device settings where you can set up your stridelength.
Via VitaDock Online:
Start your VitaDock® PC/MAC software.
Plug in the USB cable to your PC/MAC.
Connect the ViFit® Connect to the concerned contacts.
Choose the option to view your data online.
In the "Settings" section choose "ViFit".
In the section "STRIDE" you can set/change your stridelength (in cm).