Your kitten should already be house trained but as it’s in a new environment, accidents are bound to happen. When this does happen, just pick the kitten up gently and place him/her in their litter tray to remind the kitten where the tray is situated and that is where it should be going to the toilet.
Until the kitten has fully settled in with you, it is very wise to keep the tray in the same place in the house and not to move it around. You also need to make sure that there is a tray upstairs as well as downstairs because the kitten still doesn’t know where everything is yet, and if it’s upstairs and needs to go to the toilet, it won’t remember that there is a try downstairs.
It’s always helpful as well during the first few days to place the kitten in it’s tray periodically so that it’s gets used to where it is. Once it’s got the hang of it, there shouldn’t be any more accidents. Remember that if an accident occurs, never to punish the kitten, as it doesn’t know what it has done wrong.