If your tiles are flat, there are a few options to choose from:
Quadrostyle tile stickers can be used on 15cm x 15cm or 30cm x 30cm tiles. The sticker needs to fit the tile underneath exactly without covering grout lines.
15cm x 15cm Tile Stickers
30cm x 30xm Tile Stickers
Self-adhesive wall tiles are made of rigid vinyl and can be applied over any size flat wall tile, they will cover the grout lines underneath. The tiles are fully waterproof thanks to a clear membrane which prevents water getting behind the tiles making them suitable in wet areas, even shower cubicles!
3D tile stickers are ideal for covering splashback tiles and can be used over any size flat tile to cover what's underneath
3D wallpaper can be used to cover flat tiles if you use the special glue. It's ideal as a kitchen or bathroom splashback panel as it's applied horizontally as one continuous length (thus minimising joins) but can also be used in shower cubicles if the edges and joins are sealed with silicone.