back of the handle adjustment with a deep 10mm socket and ratchet.
remove the rear calipers and discs, clean off all the dust on the shoes.
The adjuster is at the bottom of the shoes and use a flat blade screw driver to wind out the adjuster, won't need to be much, a few full turns.
Slide the disc over the shoe so it feels like the inside of the disc is just skimming the shoe, then adjust up the the handle to 6-7 clicks and she should be sweet.
IF you got some vernier calipers that go to 20cm you can set the measurement to the correct specs which are 189.5 to 189.7mm.