I've looked everywhere and it doesn't appear that anyone has posted this previously so I thought I'd post a short guide on this.
To change the client certificate password of your Checkpoint Mobile Access VPN certificate, you can follow the below steps.
1. Open your Check Point Mobile Client and browse to your certificate if it isn't already populated in the field.
2. Click on the "Certificate" Icon highlighted in the below screenshot

3. Enter the current certificate password into the presented prompt as shown below

4. On the window that appears, click the "Change Password" button as shown below

5. On the "Change Certificate Password" window that appears, enter the old and new passwords as prompted and press "OK". If you've done this correctly, you will be shown the "Certificate password replaced successfully" window as below

6. You can now press "OK" to close all the windows and connect with your new password.
Easy and quick to do!
This does rely on the user proactively changing the password and doesn't force any complex rules, but it's one step in the right direction.
If anyone knows how to to configure a minimum time period to force password change, or to enforce complexity requirements, I would love to know, so please reply to this post.
Thank you for reading!