my Edge 810 dont read the HR. Is it a battery problem or something else? How can I know? To change the battery, can I do it myself or do I have to go to a center?

You don't say which heart rate monitor you have, but for almost all of them, there's a door on the back that you remove to replace the battery, usually a CR2032 coin cell, which is commonly available at almost any place that sells batteries.  Usually there is a slot that a properly sized coin will fit in to turn the door to remove it.  Generally, a large size coin fits best.  I use a Fiji 20 cent piece, but a 2 Franc coin also works well.  A large washer from a hardware store can also work.

Did it ever read the H.R.? Did you correctly sync the H.R. to your Edge? Are you wearing the strap correctly. You were not very clear with your question or even the problem.

With my 820, i had problems locating it when i searched for all sensors, i then chose to search specifically for heart rate monitors and it appeared.