Follow the below steps to create a new permission level
Navigate to Site Actions --> Site Settings --> Advanced Permissions
This page displays the complete list of user groups and their respective permission levels. Click on the group to add / remove associated permission levels
Go to Settings --> Permission Levels to create a new permission / customize an existing one.
To customize the existing permission level, click on the permission level and check / un-check the permission check boxes to match your requirement.
I am going to create a new permission level “Low Contribute”, which allow a user to add, update an item but not delete. A little bit of customization to the Contribute permission level will give us the desired functionality. Click on “Copy Permission Level” which will copy the permission level, give a name to this permission level (“Low Contribute”).
Create the new permission level by un-checking “Delete Items” and “Delete Versions” from “List Permissions” section. The following screen shows a user with “Low Contribute” permission.
Note: The security trimmed menu does not have the Delete option.