How do you replace the alternator on 2002 Nissan Altima with ac compressor in the way?

In general, to replace an engine component when another is in the way, you have to remove or move the obstructing component. However, you do not want to disconnect the ac compressor. Unbolt it and move it a little bit. Depending on the vehicle, you should then be able to remove and replace the alternator. If the lines are not flexible, then you may need to disconnect it - in that case you need to evacuate the refrigerant, reseal and purge, and replace the refrigerant - and you need a properly trained and certified mechanic to do that.