Open …\Steam\steamapps\common\SpeedRunners\Content\ (Steam: right-click the game › Manage › Browse local files)
Back up Characters\VelociTrex\animation_variant01.xnb, animation_atlas_variant01.xnb, heads.xnb, win_variant01.xnb, win_atlas_variant01.xnb and UI\MultiplayerMenu\Lobby\characterPortraits${sheetN}.xnb
Copy the zip's folders into Content\, replacing the originals (the zip keeps the game's folder layout)
Start SpeedRunners, pick Veloci-T-rex and skin variant 01. Verify integrity of game files in Steam puts the originals back any time.