You can start with following to resolve battery drain issue:
1) Check if any recently installed app is causing the problem. You can check battery usage by going in "Device Care" and clicking on "Battery" section. Uninstall any app if you find that its causing battery drain. 2) Check if your memory card is faulty or needs formatting, as sometimes read-write causes battery drain issues. 3) Check if you have good network range or if SIM card needs to be upgraded / changed. It is a good practice anyways to change SIM card (without changing the number) if it has not been changed for the last 5-6 years. 4) You can try "Clear Cache" option from Recovery menu. This will not delete any data on your phone. 5) If all of the above does not work, take backup of your phone including Contacts, Messages, WhatsApp data, etc. and Factory Reset your phone. It is recommended to do this after a major OS upgrade. This should solve most issues on your phone.
Don't charge your phone every time. Keep it between the range of 15 - 90% for better battery life. Or calibrate battery (if not already done) by draining the battery to 0%, charging to 100% (don't use your phone during this full charge cycle) and then again drain battery to 0%. This will reset the battery stats.
If everything mentioned above fails and you still face major battery drain issue, then hardware (battery) is damaged and needs to be replaced by going to official Samsung Service Center. Hope this helps 🙂