How do I increase the water level in a toilet cistern

With or without changing the water content?

With increasing the water content : if there is a "floater" just bend the arm of that one slightly upwards, till the water level reaches the preferred level. Be careful not to overdue it : danger of overflow!

Without increasing the water content : do as above, but add one or more half bricks to the cistern.

:)

How do I increase the water level in a toilet cistern
It depends upon what ballcock you have. For a regular float ball ballcock ,(see image), I set the tank level by adjusting the height of the float ball.
There are two ways to do this, 1) by adjusting the screw on top of the ballcock, **or** 2) (My personal favorite) Grasp the rod just ahead of the float ball, (do not grab the ball itself) with one hand and next to the ballcock with the other. Bend the rod up to increase the tank level and down to decrerase the level.
If you have a FliedMaster Ballcock,(see image) then pinch the metal clip and slide the can up to increase the level and down to lower it.
The tank level should be set at 1/2 inch under the top of the white overflow tube.
Good luck, Tom