By default your phone camera will capture photos in full color. Occasionally however you may need the picture in black and white. On Tecno mobile phones, you can either shoot a picture in B&W in the first place or make a color photo into black and white by applying filters or desaturating it.
Take photos in B&W on Tecno mobile
The stock AI Camera app on Tecno mobile phones come with various color filters. You can quickly switch color filters to take photos in different styles.
Fire up the Camera app on your Tecno phone, tap the Filter icon at the top right corner, then the filters will be displayed above the Shutter button. Swipe left to the right end to find and apply the Mono filter. Finally press the Shutter button to shoot the photo in black and white.

Mono filter simply removes all color from the image, similar with desaturation. They make no adjustments to contrast, brightness, or other aspects.
Apply black & white color filter in Gallery app on Tecno phone
Open up the Gallery or AI Gallery app on your Tecno mobile phone. Find and open the photo in Gallery, tap on it to show the toolbar at the bottom, choose Edit > Filter. Swipe through to find and apply the black and white color filter. On my Tecno Camon 15, the B&W filter is titled B_05. It may vary on different phone models. But you can preview the filter thumbnail to find the B&W filter.

Tap the Done button (check icon) at the top right corner, then tap on Save button also at the top right to save it as a new picture.
Convert color photo to black & white by desaturating it
The stock Gallery app on Tecno mobile phone also allow users with various editing options. For example, you can adjust brightness, contrast, color, saturation, highlight, lowlight, clarity and many more. You can change the saturation or desaturate a picture to remove all color from it and thus converting color photos to black and white on Tecno phone.
Find and open the photo in Gallery or AI Gallery app on your Tecno mobile. Tap on the photo to show the toolbar at the bottom. Choose Edit > Adjust, you will then find options like brightness, contrast, color, saturation, highlight, lowlight, clarity at the bottom. Select Saturation. Then swipe left or left on the photo to adjust its saturation. You will see the saturation value, displays on the top bar, changes at the same time. Tap on the Done button (check icon) at the top right corner to save the change.

Optionally adjust other aspects of the picture, such as brightness, contrast and so on. Finally tap on the Save button at the top right corner to save it to your Gallery as a new photo.