HOWTO? if it exists, increment it, else, insert it

I can't seem to get CASE to work for me here and every example that I see is using CASE in a completely different way.

have a table:

day(default CURRENT_DATE), user, count_of_a, count_of_b

index is ("day", user)

What is the best postgres way to handle updating this table?

I basically need to say,

if (day,user) exists, then increment count A
else insert day, user, and count_a = 1.

thank you in advance