Generally most of head units have holes, some in the corners of the front panel, like stock radio, others in the middle height on both left&right sides. They may look like slots, and also may be hidden under removable front panel small frame (if you have one) . Inserting something in (depending on a holes/slots shape) about 2-3 inches deep should unlock the cage locks.
When you will be connecting new head unit - check every wire for voltage with ignition on - wire colors probably will not match, and you don't want to burn built-in amp by connecting speaker output to +12V wire.
There are three ways to do it - good one, bad one and russian one