There are 2 ways to perform a "Local update" of your ROM. It will need specific local update firmware files, wich differ from the ones used for flashing!
FIRST METHOD: FROM THE FIRMWARE UPGRADE APP
Save the ZIP-File of the new ROM to your phone storage and select it at "Settings > About phone > Wireless Update > Menu [...] > Local updates".
SECOND METHOD: FROM THE RECOVERY MODE
0) Save the ZIP-File of the new ROM to your phone storage, preferably in the root directory.
1) Switch off your device by pressing and holding down the Power button and, when prompted, tap on Power off. If unable to tap on Power off, simply keep holding down the Power button for several seconds until the device turns off.
2) Press and HOLD Volume+ button, then, witohut releasing such button, press and HOLD the Power button until the Blackview logo appears. As soon as the Blackview logo appears, STOP HOLDING the Power button but KEEP HOLDING the Volume+ button until the wrecked android robot with the "No command" text appears.
3) Now press and HOLD Power button, then, without releasing such button, press ONLY ONCE the Volume+ button. Now release the Power button. You should see the Android system recovery options' list at the top of the screen.
4) Use the Volume buttons to highlight/select the "apply update from sdcard" option, then confirm it by pressing the Power button.
5) Use the Volume buttons to highlight/select the ZIP-File you saved on your phone at step 0, then confirm it by pressing the Power button.
6) After the "Install from sdcard complete" yellow statement appears, use the Volume buttons to highlight/select the "reboot system now" option, then confirm it by pressing the Power button.
After having booted your device to the new ROM, as always we suggest to perform a factory reset to avoid weird issues.
PAY ATTENTION: If none of the 2 methods seems to work, please flash your device to previous firmware versions as 20171219 and 20171227 and try again!