How can I increase the strength of an electromagnet?

A battery with more current capacity.

Lots and lots of wire, but much thinner wire. Use magnet wire, which has thin but high temperature insulation so more turns can fit on the armature. Wind the wire around the metal core in neat, closely spaced turns. Use a variable speed drill to wind the wire around the core.

Use an iron core material.

Use a shaped core (see source).

Test your electromagnet using a linear hall effect device to find the magnetic strength produced. Hall effect devices are cheap, and can be used with a simple circuit and a voltmeter to show field strength.

The diagram is rubbish. A few turns of thick wire across a 9V battery = dead battery.

You need a LOT of turns of THIN wire. And a 1.5V AA battery will work fine.

how can I strengthen or weaken an electromagnet?

by increasing or decreasing the current. Use a series resistor of a few ohms to reduce the current. More cells in series make it stronger.

You can easily weaken it by removing turns from the coil, but particularly if it's supplied DC, you may need to lower applied voltage until the wire can handle the increased current.

You can strengthen it by adding more turns, but may need to INCREASE the voltage applied. Or, you can make it stronger by rewinding the coil with larger gauge wire, so current can be increased.

The voltage needed will be decided by the recommended current rating for the selected winding wire, assuming continuous operation is required. For intermittent operation, the current can be considerably higher, but for short times only.

More current, more magnetic field, more turns, more magnetic field, etc.

Try get narrow ray. Very very narrow.Iiiimmm....it is.