I updated to unity 2020.1.15f1 and then I downgrated to 2019.4.15f1 becasue i had some problems.

Now that i downgrated i got this errors, should i just go to this scripts and delete the unnecessary parts

 

Use your source control software (or backup) to revert your project to before you did the upgrade.

If you do not have a backup/repo (which I am assuming you don't, since you are having to ask this question), try deleting the library folder and letting Unity rebuild it. Backup your project before attempting this.

 

This ^^

In the future, remember that Unity doesn't actually support version downgrades. After you've done the above, if you still have problems the errors are complaining about code in one or more packages. Go into the package manager and verify the package versions are appropriate for the Unity version you are now using.

When you upgrade Unity versions, the editor runs through a process of upgrading your project to the new version. What changes happen to your project during this process will depend on what has changed between Unity versions. There is no similar procedure for Unity downgrades to reverse all those changes. So using source control and/or backups is pretty important when trying out a new Unity version, or you're stuck manually figuring out how to get your project back to a working state in the older Unity version.