You can return it to the Alameda County Registrar of Voters Drop-Off Mailbox located in front of Dublin City Hall, 100 Civic Plaza, Dublin, CA 94568.
It can also be mailed to the Registrar of Voters or delivered in person to any polling place in your county no later than 8:00 PM on election day. If illness or a physical disability prevents you from returning your ballot in person, you may designate a close relative to return the ballot to the polling place.
The Registrar of Voters Office is located in the Alameda County Courthouse at:
1225 Fallon Street, Room G-1
Oakland, CA 94612