#2
The short answer is that you probably can't as there is no adjustment mechanism for it.

However intonation issues can come from old/bad strings, so you might want to change the set to see if it gets better.
#3
Intonation on an acoustic guitar is adjusted by making relief cuts in the saddle. Naturally, there's not a lot of room to work with there...
First step...Have the instrument's action adjusted properly. If the strings are too far from the fretboard at the higher frets, you'll pull them slightly sharp just by fretting.
With the action adjusted properly, you can then check the intonation and determine if it's off and by how much.
If it's just a few "cents" (100ths of a tone) off, then a technician can normally cut the saddle on offending strings so as to move the contact point back or forward a small amount.
If the intonation is really off, the only thing to do is to re-cut the saddle groove, which may involve replacing the entire bridge...
It might be better to save up for a higher-quality instrument.
#4
Different brands of nylon strings and different tensions will all intonate slightly differently. The search to find one particular brand and tension that settles properly on your guitar may be a long one. It took me quite some time, and while most were OK, only a couple were spot on after settling.
PRS SC245
Various Strats
Polytone Mini Brute
Koch Studiotone XL
Quilter OD200, 101 Reverb and Mini
1958 National lap steel
Eastman El Rey 1