this is a bit confusing because you ask about filling tanks at altitude and then about diving at altitude.
Filling tanks is filling tanks, the compressors largely don't care about what altitude they're being used at. They are less efficient at altitude for the same reason a naturally aspirated car loses power at altitude, but air pumped at sea level and air pumped at 6k ft is still air and 3000psi gauge in the tank is still 3000psi gauge in the tank regardless of altitude. We assume that a scuba tank is a rigid vessel and therefor is not subject to changes in atmospheric pressure. I do say assume because it's not perfectly rigid as it stretches a bit when filled, but it's functionally rigid and atmospheric pressure is certainly not enough to cause any deformation.
Now, while
@lostsheep did not provide a particularly useful response and that first post is straight up trolling, I have to agree that you did not provide adequate information on either your compressor or computer to have us make any real recommendations.
That all said, this is how dive computers work. They read ambient pressure and then use that ambient pressure to run the math portion of the dive. With open circuit gases you do not set partial pressures of gas but rather fractions of gas and the computer will use the ambient pressure reading to convert the fraction of gas that you put in to a partial pressure of gas that it displays and uses for decompression calculations. Do NOT make any adjustments to the gas mixtures as a result of diving at altitude, the gas mixtures are either exactly what is analyzed out of the tanks or rounded to standard gases from the actual analysis, i.e. 31% or 33% can just stay at 32% which is part of the whole measure with a micrometer, mark with chalk, cut with an axe.
As far as what else you need to do to safely dive at altitude, you really need to either take a course *which I personally think is not actually all that useful as the actual diving portion is irrelevant* but you at least need to read the course material and then to LostSheeps point, RTFM on your computer as the computers manual will tell you what if anything you need to do to use it at altitude.
