Restrict users from Deletion

cad-one 

There is no SharePoint default permission level which allows add/edit but not delete existing files.

Workaround:

You need to create a custom Permission level in SharePoint. This will be a copy of your existing permission level minus Delete Items permission. Follow below steps to achieve this:

1. Click Site settings > (Go to top level site settings) > Users and Permissions > Site permissions.
2. Click Permission Level, and we can customize a new permission level based on the existing ones.
3. Open the Edit permission level, and click Copy Permission Level on the bottom.
4. Give the name for the new permission level, and uncheck the Delete Items options. You can also remove other permissions that are unnecessary, such as Delete Versions.
5. Click Create to create the permission level.
6. Use this permission level to assign the permissions to users

Reference: SharePoint online site permissions add but not delete 

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