Cenvat credit reverse - excise

Dear Sir

we purchased 100 kgs material  in the month of June, same month we taken cenvat credit in excise return.

in that material we used 50 kgs for Excisable goods in the june month and remaining used for Excise exempt goods in july month. my question is how can reverse cenvat credit in july month and where can i take in excise return.

please help me.

regards

Arunkumar

Finance Executive