The simple answer of how to defrost a freezer is take everything out, unplug it and prop open the door. If your freezer slowly begins to fail it is very possible you need to defrost it. Many freezer have an auto defrost cycle built into their schedule. If your unit is supposed to auto defrost and you find yourself needing to defrost the coils there is a problem. However if the frost is on the walls rather than the cooling coils you are in good shape.
If your freezer has a fan in it and the coils in front of or behind the fan have ice on them you likely have a problem with your defrost cycle and need a technician. You should read the manual if your freezer calls for a periodic manual defrost than the typical process of take everything out unplug it and prop open the door applies. You may be tempted to use a hair dryer or chisel to speed the process. You are only improving your odds of screwing something up by getting in a hurry. A pro-tip for deep freezers or freezers that open from the top. They will typically have a plug which you can remove for this process.
The one advanced thing you might do to improve the efficiency and life of your freezer is clean the coils with coil cleaner after it has defrosted. Your should however read another blog about coil cleaning before endeavoring to clean your freezers coils. They can be irreparably damaged by aggressive cleaning.
In conclusion, don’t be a hero. Just unplug it and open it up. Clean up the water mess when it is done defrosting. If it has a stinky mold smell you can use a 5% bleach solution but again don't go crazy with chemicals.