My air conditioner is still "working", and when I go downstairs it's
cooler and I can feel the difference. The problem is that for some
reason there is not enough cold air coming out to cool my upstairs.
It's been working fine for the past 3 years, so this is a recent
problem that has developed.

I thought it might be the filters, so I've changed the filter upstairs
where the air goes into the HVAC unit, and the filter in the HVAC unit
itself that resides in my HVAC room next to my garage. I tried
resetting the breakers for my air conditioning unit, and I see the
compressor fan turning outstairs.

It's summer time, so the temperature can easily get into the 90's. I
have my thermostat set for 75 degrees, with the "Cool" selection (as
opposed to "Heat"), and with the fan set to Automatic. When I place
my hand over the vent upstairs I can feel the cool air coming out, but
it doesn't seem to be as strong as it used to be...and perhaps not as
cold. As the upstairs temperature never seems to hit the 75 degree
mark, the unit keeps going and going and going which surely can't be
good for my electricity bills.

Is there anything else I can try? What do I do next?

Thanks
Sam