Concur, simplest way is to replace fan.
Baring that, Yes they can be replaced. Not knowing your experience/knowledge - They are polarity sensitive so need to observe +/-. No damage if reversed, they just do not light up. Exception - ones used in an AC circuit which will light on one alternation and off the other alternation, NOT normally the case so will not go into constraints
Repacement LED come in several flavours, most are approv 2 volts and draw 20 mills (supper bright about 30 mills). There are also one that operate at 5, 6, & 12 V so need to get the correct voltage. Ones above 2 volts use a series resistor either internal or external to the LED. Example a 12 V led wil have a series resistor of approx 600 ohms.