The United States Postal Service will exchange any
damaged stamps that have been wet
and stuck together providing they have not been
used; whether full sheets, plate blocks or single
stamps.

Their instructions are as follows:

Soak the stamps in order to peel them apart, then
glue them to a sheet of paper (Scotch tape is ok).

They will then exchange your damaged stamps for
new stamps.

There is no such thing as obsolete postage stamps.
Every United States postage stamp that was made to
be used for postage is legal to use no matter
how old it is.

JB