Medicine needs to be set up in your PigVision software before you are able to use it in PigVision Mobile Sows.
Once set up and the data is synced to the mobile app, you will see a new "Retreatment" option on the home-page. This is used for subsequent treatments after the initial one in the days that follow.
Click on the animal, which you need to treat again.
You can also retreat several animals by clicking on "Select" in the top. Here, you can choose which animals by selecting each of them or you can select all: