As general points, gather evidence or documents (while everything is fresh in your mind) that shows how your targets have changed (particularly if there is any evidence to suggest the new targets are unreasonable, unachievable or out of your control), or task lists/Job Descriptions showing the changes to responsibilities.
If you would like to exit your employment under a negotiated agreement, please refer to the specific FAQ on this.
If you feel unhappy with the way you are being treated by your employer, and want to raise this so it can be resolved, please refer to the specific FAQ on this.