It would stand to reason that you must shoot below your current average to make it go down. 62 or better for 18 holes and/or 31 for 9 holes.
Fortunately, it's not that hard.
First of all, start to think in gross scores rather than +-Par, and double 9 hole scores for this calculation. Thus you are talking about 67 to 73, from your last scores.
The average is computed from a certain number of the best scores in one tier.
Once an average is "saturated" like yours is, only those scores will modify it which push a higher one out. This may be a 68 / 34 now, but quite certainly not a 70.
Just watch which scores will diminuish the average, this shows you where you are.