It appears someone has changed my pass code to my phone how can I fix this?

You will need to restore the device using a computer. The process will erase the device. Later you can restore your data from a recent back-up. Instructions here…

If you forgot your iPhone passcode - Apple Support

During this process, you will be able to set a new Passcode.

jednikral wrote:

How can I change my passcode?

If you forgot your iPhone passcode - Apple Support

No one can change the passcode of your phone without knowing the current passcode.