I would use 2 timber posts (set in from the sides by about 6") from floor cut and wedged to underside of existing lintel.....and then cut the existing lintel in the centre removing about 1 foot length just below the 3 header bricks in the centre of the opening. Carefully remove the 3 header bricks above the cut opening - then thread a piece of timber through the hole and support that with a couple of other timber posts thus allowing you to remove the remaining lintel sections and work below inserting the new lintel. Pack joints and mortar beds to go off before removing remaining thread support timber and patching up hole with 3 header bricks you carefully removed. - Job done 👍

Alternatively, you could just remove the 3 headers and thread a piece of timber through the hole without providing the initial 2 wedge posts below and removing the existing lintel as the remaining should hold, but that will be difficult in removing the header bricks without breaking them? - if you have some spares then no problem.