You may find that when you start using heavier eyepieces or perhaps a camera with your little NexStar that your scope slips in altitude. The best solution for this problem is to balance the scope with weight added to the other end of the optical tube. If the scope tends to move easily with only a little pressure on the end of the optical tube, balance might not be enough and you will need to adjust the altitude axis.
Open the tube ring and set the optical tube aside. Tighten the large nut found inside the tube ring by 1/4 turn. If it still slips try another 1/4 turn. If that doesn't do it you may have a problem with excess grease at the friction point.
Remove the nut, noting the order of the washers. Remove the tube ring and you will find a large gear and a fiber washer. At a minimum you must remove all grease from the fiber washer and the part of the gear where the two meet. You might use a small amount of acetone (fingernail polish remover will work in a pinch, but it tends to have other ingredients as well) and cotton swabs or paper towel. If the gear comes off, you will find three nylon disks on the other side. These must go back into their holders in the fork arm.
If you are more ambitious, you may decide to remove all the existing grease from the gear, fiber washer, nylon disks and fork arm. Then you can re-grease the teeth of the gear and the nylon disks with a good quality lithium grease.
After replacing all the parts, you need to adjust the friction setting. Try moving the OTA up and down by pushing on the end of the OTA. It should require moderate force to move the OTA. If it moves easily, tighten the nut a quarter turn and try again. Continue tightening or loosening until you feel the OTA will stay in place when accidentally pushed. Keep in mind that too tight can result in added wear on the altitude motor/gears and poor battery performance.