Changing a bicycle tyre can be a tricky task if not done correctly, so if you're not experienced, it's always advisable to rely on a skilled mechanic to do it. However, if you prefer a DIY approach, here is a step-by-step guide on how to proceed:

Remove the wheel: If the bicycle has traditional rim brakes, widen the brake pads using the appropriate system (each brake brand has its own system). Loosen the quick release on the hub and remove the wheel.
Deflate the tyre: Remove the valve cap, unscrew it, then press the central knob to release all the air from the inner tube.
Remove the tyre with tyre levers: Use tyre levers (or tyre pry bars) to lift the bead (or edge) of the tyre off the rim. Insert the first lever between the rim and the tyre, being careful not to damage the inner tube if present. Avoid trapping the inner tube between the lever and the bead, as it could tear.
Completely remove the bead: Repeat the process a few centimetres away if necessary, until the bead is loose enough to be easily removed by hand from the rim.
Remove the other side of the tyre: Flip the wheel and repeat the process for the other side of the tyre. This part usually requires less force.
Check the nipple tape and tubeless system: Check that the nipple tape (if present) or the tubeless-ready system is in good condition. If damaged, replace it according to the rim manufacturer's instructions.
Install the new tyre: Start mounting the new tyre from one side. If the tyre has an inner tube, insert the slightly inflated tube to prevent damage. Make sure the bead is properly positioned in the rim's channel, and proceed to fit the opposite bead. You may need to use the levers for this, but be careful not to pinch the inner tube. If you're mounting a tubeless tyre, add the sealant before finishing the installation of the bead, or through the valve if the valve core is removable.
Inflate and check bead positioning: Inflate the tyre until the bead is correctly seated around the entire circumference of the wheel (rim line visible). If using a tubeless system, rotate the wheel to ensure the sealant is evenly distributed inside.
Adjust the pressure and reassemble the wheel: Set the tyre to the desired pressure for use and reattach the wheel to the bicycle, following the reverse steps for quick release or thru-axle installation and brake assembly.