This is a DIY job, if you take your time. Go to the Dell site and look up the model number of your computer. If your Dell support isn't current, don't worry, you'll still be able to download the info you need.
You can open the computer case and look for the battery physically. It's the disk shaped kind. It's mounted in a round plastic holder. Don't pry it out, there's always some protruding piece to push on and it will pop out, like toast.
After installing the new battery, you have to go to the bios and reset the time and date. Again, check the dell site to see how you navigate that. You'll be using the arrow keys, enter key and escape key. Very easy once you see how to do it. Good luck.