Q.I might have over fertilized my new guava tree.
Brought new 3 foot guava tree indoors for winter, grew great outside for Summer. I fertilized it with a dash of iron and some all purpose Algo-Flash. Leaves started dropping a week later. Now the edges are turning brown and dropping 3-4 a day. Should I try to rinse it by letting water run through it for an hour or 2 to drain out the bottom, or should I remove it from the pot and replace the potting soil with new potting mix? Which would be less traumatic for the plant? Even new shoots are turning brown and falling off.

Leaching the excess fertilizer with water is less traumatic than re-potting at this point, which would cause some root loss.
I don't think you need to run water through for an hour or two, just water heavily a time or two and hope it refoliates.
I'm not familiar with Agloflash and I can't find a list of ingredients, but I prefer a slow release organic fertilizer over concentrated chemisty, no matter what the claims.
https://www.domyown.com/dr-earth-organic-all-purpose-fertilizer-p-9485.html
Even then, following the directions for application rate is important, guess-work gets your plants in trouble.