I just bought a Giant TCX SLR 2, but I still have money to spend. Which parts should I upgrade? I am new to bicycling. I bought this bike because it had good reviews and ratings and it fits the area where I am living. I am not intending to use this in a cyclocross race, but maybe in the future. I still have money to spend, so which parts of the bike should I upgrade for the bike to function even better?
A good investment for someone new to cycling who starts with such a sporty bike is to get a good bike fit. Based on the findings of the fit a part that maybe has to be replaced, is the stem. As these come at different lengths but are not adjustable.

You need to get pedals as well. You may choose between flat pedals, clip-less road pedals and clip-less mountain bike pedals. Each have their advantages as well as disadvantages on a cyclo-cross bike. For the clip-less pedals compatible shoes are needed.

Criggie mentioned in a comment useful tools and accessories that you might need depending on your intended use.

But first of all, get accustomed to the bike and find out what you like to use it for, as this changes everything.

For instance, if you use it to practice CX on a lawn, five minutes walk from your home, you will not need spare tubes and tools on your bike. Conversely, if you ride long remote gravel roads, a days march from the next settlement, you will need to be very well equipped for all kinds of field repairs. In addition, each use case requires a distinctly different bike fit.