Why changing a password is so difficult.

You can change your Apple ID password following the steps at the following resource:

Change your Apple ID password – Apple Support (UK)

This should be a simple process, you shouldn't come into any difficulties with this. What specific error are you having? Please remember that as this is a user-to-user forum, you should keep any personal information out of your reply.

You can't use a password that you have used in the last year.