var skin_num;

//We need to increase the skin number

function set_skinnum()
{
if(skin_num <= 3)
{
skin_num +=1;
}
}
//Skin Bitmaps

BMAP* FirstSKIN = <skin1.bmp>;
BMAP* SecondSKIN = <skin2.bmp>;
BMAP* ThirdSKIN = <skin3.bmp>;

//Now,we will assign a skin to the model
function set_entskin(ENTITY* ent)
{
while(skin_num == 1)
{
ent_setskin(ent,FirstSKIN,1);
}
while(skin_num == 2)
{
ent_setskin(ent,SecondSKIN,1);
}
while(skin_num == 3)
{
ent_setskin(ent,ThirdSKIN,1);
}
}
//Assign set_entskin() to your entity

//Now you need to call set_skinnum() in main() function
function main()
{
...
on_e = set_skinnum;
...
}