Worm farm too alkaline

My worm farm is too alkaline - between 9.0 & 9.5.
How can I lower it to more neutral safely for the worms.

Thankyou
Colleen

answer from BigTex Worms

Assuming you are using a DIGITAL PH meter, cause the other ones can be wrong.... you can simply add some lemon or lime peels OR coffee grounds to your bin throughout to lower the ph. Do this and check it again in 2-3 days.