I'm guessing you're
An ace in a deck of cards.
I can be found in an alphanumeric town,
Specific cards in the deck are usually denoted by both letters and numbers, e.g. H for hearts, D for diamonds, etc
Mostly above one who has a crown.
In a sorted deck, the Ace is usually 'above' the King (who wears a crown in the picture)
If you think I'm high, I can also go down.
In blackjack, and many other games, the ace can denote a value of 1 or 11 depending on the situation
And I can be replaced, but only by a clown.
In some card games, the Joker (aka Clown) trumps the Ace (e.g. in some Poker games, Joker can denote Any card)
You don't know it's me until I'm turned around,
This was the giveaway, you don't know if a card is an Ace without turning it over
Unless you're a deceiver and the town is a battleground. Edited
Most likely referring to Ace in the hole, in which on the battleground (e.g. game of Poker), a special weapon (i.e. our Ace) is kept hidden by the deceiver (e.g. a player) which can be used to win the round