The lamps/tubes are easily available from Poundshop or similar. Usually one screw at the top of the lamp unit, undo, and push the old tube to the left or right to extract, and replace with the new one. Easy peasy. Make sure the electric supply is turned off first ! The dud tube can usually be recognised by the black deposit inside the glass. If that doesn't do the trick then a complete new unit is needed.
OnePlus 5T review: Hands-on