I recently got my uke and am really enjoying it , it’s a Kmise concert is there any advantage putting better quality strings on it my only concern is the second string from the top sounds a bit flat .
Strings are the least expensive say to change and often improve the sound of a ukulele. Start experimenting with different types of strings. I change every 1-3 months because I play a lot. As soon as a string starts to go flat I change the set.
With cheap strings, intonation can be a problem. You get the open string in tune and then when you fret it, it's off. If all the strings are off then setup needs investigating. If it's just one string, then the string is probably at fault. Changing to decent quality strings can make a big difference.
I use mainly Living Water or Worth Clear fluorocarbon strings. Both excellent quality.