To remove a bound, set it to + or -. Interactively, use the identifiers inf and -inf instead of the symbols. To change the upper bound on x2 back to +, use the one line command:
inf
-inf
x2
change bounds x2 u inf
You receive the message:
New bounds on variable 'x2': The indicated variable is >= 0.
The bound is now the same as it was when the problem was originally entered.