How can I change/up my goals on the activity app ?

The seller of the watch must take care of that. The person must unpair his / her Apple Watch before it is passed on to you.

This can be done in the Watch app. When you turn the clock on, there will be a startup screen asking you to pair the clock with an iPhone. If you see that screen, then the watch is no longer associated with the seller's Apple ID.

You can read how to Unpair Apple Watch here:

https://support.apple.com/en-us/HT204568

You need to do this on your Apple Watch.

Find your activity rings, long press / force touch, and select Change Move Goals.