Next & Previous Buttons

I was wanting to customise the way that the Next/Previous buttons work within the Portfolio and Service pages of my clients website. My developer has set it up so that the services section is a category of the portfolio section (so that it can have the same format and be updated regularly). So as a result when you scroll through the portfolio it also includes the services pages. Are you able to stop this? The way they currently work is also a back to front in that they when you click on the right hand side (which is normally “next”) it takes you to the previous item? Are you able to customise these next and previous buttons and also separate the services category from the portfolio?

Cheers
Julia