If they are Remis blinds there should be 2 adjusters on the top left end.
Use a screwdriver to push the adjuster inwards to release the lock and turn clockwise to tighten and the still with driver in the slot bring the adjuster back out to lock in place.
If you just take the driver out, the blind wil lose all its tension and you have to start again,,,,,,,,,,,but you soon learn !!!