To properly update the modpack you're playing, it depends on the launcher you are using. This video can explains it on CurseForge launcher, and the same method can be used on other launchers as well, but it's not necessary to use this method on others.
-
CurseForge - See above video
-
PrismLauncher -> Select your instance, and click "Edit" on the right. the 3rd tab down will say "CurseForge", click on it, and there will be a dropdown menu, if not already selected, the latest version of the pack will be at the top of it. click "Update Pack". Sometimes there will be new mods to manually download due to liscensing issues, click "Open Missing" and it will automatically download and checked, and copied to the instance folder. It's safe to delete these mods in your downloads folder once done. Once all are detected, the "Open Missing" will be grayed out. You can click 'Continue" at this point, and it will finish the update process and tell you when its successful.
-
ATLauncher - Under the "Instances" tab, find the pack you would like to update. there will be an "Update" button. There will be a dropdown menu, and if not already selected, the latest version of the pack will be at the top of it to select. Click "Update" once selected. Sometimes there will be new mods to manually download due to licensing issues, click "Open Missing" and it will automatically download and checked, and copied to the instance folder. It's safe to delete these mods in your downloads folder once done. Once all are detected, You can click "OK ", and it will finish the update process.