I'm a database newby and I need a little help

I have an access database with the following fields:

ID, ParentID, Level

Level and ParentID are filled with numbers Level is filled with zero's.

I want to have a loop that looks at every record an checks if the ID of that record matches a ParentID in any of te other records if this is the case the Level field of that record should be set to 1

Can anybody help me with this please ?