What happens when a military token is placed on a conquered location during a Competitive game?
The Conquest Marker remains
You couldn't find any rules about unconquering the location, because you do not do so. This is supported by several rules in the rulebook. Page 16, under Resistance Counters

Occasionally a card will call for one or more Resistance Counters to be placed onto the board. [...]
Moving Into a Space with Resistance Counters
If you move Aliens onto a Space with one or more Resistance Counters, you will need to Fight those Resistance Counters during your Battle Phase in addition to the normal Resistance drawn for that Location. If the Location has already been conquered (and has a Conquest Marker on it), you must still fight the Resistance Counters.

So, nothing happens to your Conquest Marker, but if you or an opponent enter the Location they will have to fight the Resistance Token. This is further supported by the rules for stealing Terror Points from an opponent, replacing their Conquest Marker with your own, on page 17 under:

Stealing Conquests
If a player ever has one or more Aliens in a Location with an opponent’s Conquest Marker during their own Battle Phase (and there are no enemies present), that player may simply replace the opponent’s Conquest Marker with their own, stealing the glory and the Terror Points for themselves. Note that this swap of markers only happens during your Battle Phase and only if there are no enemies currently at that Location (Alien or Resistance). Also note that this only happens in the Battle Phase, so simply moving through a Location with another player’s Conquest Marker during your Action Phase will NOT swap it for your own.