Can I change my large chainring without changing the entire crank?
The TX-50 is a tourney level part (bottom of the barrel). If the bike is spec'd with a Tourney derailleur, it is most likely spec'd with a crankset which does not have replaceable chainrings, and you'd need to put on a new crank.

As for chainring size, I'm not sure a TX-50 can take a 48t chainring (likely not - you'd have to look at the data sheet, which I also can't find), but most bikes which such a part would be mounted on would have problems with either chainline or actually getting the front derailleur up high enough to clear the chainrings for a 48t.

Your options for increasing speed generally on mountain bikes (or bikes with mountain-type groups on them) are:

1) Slick tires

2) Locking out your fork/rigid fork

3) possibly a smaller cassette

4) (Most importantly) Bring up your cadence.

If you really want to go faster on roads, look at a different (road) bike (possibly on the used market).