I can't seem to find where to replace the filter on my heating and air conditioning unit. We've lived here a year and haven't replaced it. Starting to have real bad allergies... wanted to see if filter change would fix.

Wow a year, yes filter change is needed bad allergies or not
Is hard to say for sure,but it should be somewhere in the return air part of your system either close to the blower where it is pulling in return air/ or might be in or behind the return air grill
After a year it might be rotted away and nowhere to find
Goodluck