PSI on tires/Speed

Tires:
Okay, I’m a little confuse on this and I’m hoping someone can steer me in the right direction. There is a sticker on the frame of the ATV that states, tires PSI is 5. However, after reading the side walls of the tires it states 10 PSI. The reason why I’m asking is because with these stock tires are only 4 ply I really don’t want to hurt them, but again, I would like to put air in my tires as their are all reading between 3.5 and 4.5.

Speed:
Another part of being confused on about the breaking in the ATV and being in "LOW" or "HIGH". Dealership states 15 mph or below in low gear. Over 15 mph it’s high. I purchase this ATV to make it into a work horse around my horse farm. Most of the time I keep it in low, and usually stay below 15 MPH, but it seems the fan comes on more often. When I go out into the pasture (6 acres), if I have the need to go faster (for about .5 miles), yea I’ll jump it up into a high. But usually what is the general rule here.

Thanks