Remove the valve core to deflate the dirt bike tube, while the air is draining loosen the rim lock nut. While you are at the dirt bike rim lock grab a tyre lever and begin breaking the bead and working your way around the tyre. To do this insert the tyre lever lightly between the rim and the tyre and lever down towards the centre of the tyre.
Once the bead is broken use one tyre lever to gently lift the tyre over the outside edge of the rim. Be careful not to push the lever in too deep or you may damage the tyre tube, or too shallow your tyre lever may slip and you will bash your knuckles. Once you have the first part of the tyre over the rim, slip the end of the tyre lever in under the dirt bike sprocket/ dirt bike brake disc to hold it there. Then use your second tyre lever to begin moving around the rest of the tyre lifting the bead over the rim.
Once the side of the tyre is done, flip the tyre over and repeat the process on the other side of the tyre. Once the tyre beads are one the outsides of the rim find the opposite side of the wheels rim lock and push the tyre against the rim towards the rim lock. This will create a gap on the rim lock side. Use a tyre lever at the rim lock to insert and push down so the whole tyre pops off the rim to one side. Then use your hands to push the tyre the rest of the way off. This will be a lot easier with a tyre change stand but improvise if you need to and your tyre should come off easily.
Once the tyre is off take a quick look and inspect the condition of the rim and rim tape. Use a wire brush and some contact cleaner to clean the inside of wall of the rim where the rubber bead sits. Over time dirt and debris can build up so cleaning it will ensure an even and stronger lock of the tyre to the rim.