Get the wheel off and the spacers out. If you've never had the bearings out, you need to understand how the hub is setup. Starting from one side, there's a grease seal, a snap ring, the bearing, a long aluminum spacer that runs from bearing to bearing, the other bearing and the other seal. The spacer is usually tight and you need to move it a bit to be able to drive the bearing out. 

Remove the seals, which can get trashed in the process so have some spares around. You can roll the seal lip over and get the springs out of the seal before removing them to try and salvage them. Then remove the snap ring.

I have a 2' long 1/2" soft steel rod that I got at a farm supply store. I put that into the hub far enough to contact the spacer just above the opposite side bearing. Then use it like a crowbar to push the spacer away from the opposite side bearing enough to be able to get the rod to contact the bearing. Then I whack the bearing with the rod and hammer. The spacer should move easier after that but work your way around the bearing that way until it's out. The spacer should drop right out. Flip the tire over and drive the other bearing out. 

Now take a sharp knife or tiny flat screwdriver and pop the seal off from the inside race on both sides. Clean the bearing if it's really ugly and/or pack grease into it. The way I pack grease in is to put a big glob in my palm and grab the bearing with my opposite hand, index finger through the middle. Hold the bearing about 30 degrees from you flat palm and you just kind of scrape a little grease at a time off the glob and down against your palm. That forces the grease up into the bearing. You'll see it coming through from the top after a few scrapes. Keep rotating the bearing until it's all greased. I use BelRay waterproof grease, FWIW.

Put the seals back on after wiping any excess grease away. I leave as much grease in them as they'll take. This is not a high speed bearing application so you can leave them packed with no problem.

To drive them in, I start them with a hammer just tapping around the outer race until the are close to flush with the hub. Then I use a large socket and work around the outside edge to drive them home.

I know the common wisdom is that if you drive a bearing out by hammering on the inner race, you have to replace the bearing. I've been doing this for a lot of years with never a problem so I disagree with that theory. 

If the bearing is rusty or feels gritty when you turn it, trash it. Bearings are pretty cheap at the local bearing houses usually. For a quick lube in between the above full service, you can pop the grease seals and the snap ring off, pop the seal up from the outside of the bearing and force some more grease in.