To measure the wear on your tyres and to know when they need replacing, you need to inspect the tread. The tread comes into direct contact with the ground you are cycling on and therefore determines your tyre's performance.

A tyre needs to be replaced when the centre knobs are worn down to the same height as the side knobs.

To remove your tyre, deflate the inner tube until no more air comes out.
Once deflated, use a tyre lever to remove the tyre from the rim.

Make sure the arrow on the sidewall is pointing in the direction of travel.
To put your tyre back on, place the bead (the rigid part that determines the shape of your tyre) on the rim.
Once you have done this, inflate your inner tube by approximately 30% before fitting it to the rim (inflating it beforehand reduces the risk of pinching the inner tube).

Next, fit the second bead to the rim, starting at the opposite side to the valve.

Inflate your tyre carefully. The recommended pressure is indicated on the side of your tyre as well as on its product sheet (available online).
Also, before each outing, make sure that your tyres are at the correct pressure for your ride.

When inflating your tyre, make sure you inflate it to the suitable pressure for the terrain you will be riding on and, above all, the weather conditions you will be riding in.
When riding in rainy weather, lower the tyre pressure to increase the tyre's grip. On the other hand, when riding in dry weather, you can increase the pressure for better performance. Also, before each outing, make sure that your tyres are at the correct pressure for your ride.