How to update plain bathroom tile?

You could do vinyl design on it, but I would suggest stenciling it with paint. Here is how my friend, Mary did her floor: https://lifeatbellaterra.com/updatestenciling-a-bathroom-floor/

I also agree with the paint idea but make sure you use paint specific for a wet purpose. I know Rustoleum makes tub and tile paint that I've heard really good things about. Here's some more info: https://www.youtube.com/watch?v=qVs0QJcGV1E

You can paint it and the best type is an epoxy paint.

THey make stickers for the purpose exactly. You can search on Etsy. You can also paint the tile using an oil based paint.

Paint with Rustoleum tub and tile paint. Stencil patterns with acrylic paint and seal with Marine grade poly. Vinyl decals can be purchased. Walmart, Menard's, Home Depot, Amazon

Here are 4 ways to update the tile:

https://homeguides.sfgate.com/way-update-ceramic-tile-bathroom-removing-tile-104944.html

you can paint it here is the info to do that

https://www.youtube.com/watch?v=IjcYo_YA8Zo

Paint the tile in a border just using one or two rows then stencil it with your favorite design. Paint stores all sell paint made just for tile.

The easiest and most flexible way to change your 4x4" tiles is to add decals or stickers. Here's a few .....

https://www.amazon.com/tile-stickers-4x4/s?k=tile+stickers+4x4

Concrete over them. Tinted or natural

Some interesting and creative ideas here:

https://www.apartmenttherapy.com/easy-ways-to-update-your-kitchen-backsplash-243255

Even tough it is done on a backsplash, there is no reason why it would not work on your bathroom tiles. Best to you.

Here's one idea that might help - https://lifeatbellaterra.com/stenciling-a-tile-floor/

You can paint tile! Yes you can, use a primer and a chalk based paint like Fusion Mineral paint. You can then stencil a design on top.

You could add an accent tile if you don't want to do something extensive. It made so much difference when I added it to this white 4" tile.

What about stenciling it

Hello, I have personally not done this exciting refurb project —but here’s alink that might be helpful.

https://todayshomeowner.com/video/painting-ceramic-tile-walls/

You can paint the tiles https://www.homedepot.com/c/ah/how-to-paint-tiles/9ba683603be9fa5395fab90142305776

You can use a tile painting kit from your local home improvement store. I wouldn't try much else as it'd be likely to fail causing you far more headaches and expense with all that water exposure.

You can paint it? https://www.rustoleum.com/project-catalog/bathroom/rustoleum-home-bathroom-floor/