Ask the owner if you can change it.
In the Netherlands it is quite common to repaint/paper your home, even if it is a rented one. We pay for it ourselves. Always ask the owner where his/her boundaries are (color, material). If possible, add some lines to your contract, so no stange faces afterwards. Here it can be agreed to boldly paper/paint whatever you want, but when you leave the home, you paint it white, or have a meeting with the new rental if they want to re-do it too.

I agree with rinqreation about first asking the owner.
Failing that, I have used many solutions for clients.
1. Attach a tall rail to the top of the ceiling and then using tacks, add lightweight colourful fabrics that drape down.
2 Buy plywood in large rectangles, light padding and buy shot silk or some other fabric. Staplegun the fabric over the padding and then hang these together to create a kind of panelling effect. You an play with height and width but divide the panels fairly evenly along the wallspace and from ceiling to ground. (see below)
3. Buy 2x4 wood and create frames, ditto stapling the fabric to them and then wedge them up to the ceiling and to the floor
4. I have even bought large strips of paint canvas and painted it using the same system as 3. so the client got their painted walls...
Food for thought....

You could put a modern wall paper over it. But if you want to take it down make sure it doesn't take off the drywall or plaster with it. That can add cost to repair. A professional painter would be able to take it off easily and paint. But yes consult your landlord first.

Hi---don't tear it down, the walls will probably be a mess. Instead, ask the owner if you can paint it. Prime first---then paint it. CheerS!

You don't say if it is one wall, a whole room or the whole house. Permission from the landlord is needed to remove, repaper, or paint over it. Paint is the easiest option. If it has a texture, it could look really great painted.

If it is one wall only, cover it with fabric (like burlap) stapled to the wall, or install bookcases and/or an art wall over it. The burlap is removeable and reusable when you decide to leave.

Many landlords are amenable to changes if the work is of a higher quality and makes the apartment more marketable in the future. Talk to the owner or property management company to find out what is possible.