Any changes to your snippet must be done before your campaign launches.
On TikTok you promote a snippet not the full song. Most videos are 15 seconds long, so choose the part of your song that is the catchiest, most memorable, or that fits the concept you are looking to push.
This could mean showcasing certain lyrics, or a portion of the instrumental you think TikTokers would love to dance to.
Customizing your snippet needs to be done through your distributor. Or you can upload a video manually. More details here.
Note: Depending on your distributor, they may send a 15, 30, or 60 second clip to TikTok. However, most TikTokers will use only the first 15 seconds of your clip.