How as the respondent how can I change my already filed divorce response from uncontested to contested?

It is likely not worth your time to file an amended Answer/Response as there really is no benefit to contesting the divorce. You can however file an amended answer/response. The method for doing this is a bit different from county to county, but you can call the court clerks and ask them what paperwork you need to file an amended response in your courthouse. If you are now contesting the settlement (the only paperwork that a respondent should take part in filing in an uncontested divorce),

Check if your court has a self-help center where they could look at the actual paperwork and give you information to decide what the situation is and whether it's worth your time and effort to make the changes.

You can amend your court documents using 2 methods. One, is to enter into a written stipulation with the other party to file an amended document. Two, is to file a motion or an Order to Show Cause to request an order for leave of court to file an amended document.