Hotel accommodation: I booked a room, originally intended to stay for only one day, but later it changed. Can I renew it for another day before I check out?

No problem, as long as you don't check out, you can naturally continue to check in. On the contrary, if you want to check out, there will be restrictions: half price after noon 12 (in some cities 14) and full price after 18. In other words, no matter when you check in, you must check out before noon 12, otherwise you will be charged at half price (some cities 14) and full price after noon 18.