Based on the current records in a form deduct a record in a table

[Rani VSPL via AccessMonster.com]

Hi
I have a table called PartsInstall and another table called PartsRepair.
Based on the data in the Form PartsRepair i need to update PartsInstall
Table. i.e Suppose a part say NMC01 is installed in a vehicle called UG001.
Now when the same part is sent for repair then it must be automatically
deducted from the PartsInstall table. How to fetch the only record
concerned to the part NMC01 in the form PartsRepair whose Data source is
table PartsRepair .I hope I've made myself clear.
Any help would be highly appreciated
Thanks