Trying to change plug from a 3pronge hookup to a 4.The dryer only has 3 wires to hook up extenstion plug has white,red,black,green ...what do i do

It is your responsibility

To contact a qualified electrical installer.

To be sure that the electrical connection is adequate and in
conformance with the National Electrical Code, ANSI/NFPA
70-latest edition and all local codes and ordinances.
The National Electric Code requires a 4-wire supply
connection for homes built after 1996, dryer circuits involved
in remodeling after 1996, and all mobile home installations.
A copy of the above code standards can be obtained from:
National Fire Protection Association, One Batterymarch Park,
Quincy, MA 02269.

To supply the required 3 or 4 wire, single phase, 120/240 volt,
60 Hz., AC only electrical supply (or 3 or 4 wire, 120/208 volt
electrical supply, if specified on the serial/rating plate) on a
separate 30-amp circuit, fused on both sides of the line. A
time-delay fuse or circuit breaker is recommended. Connect
to an individual branch circuit. Do not have a fuse in the
neutral or grounding circuit.

Do not use an extension cord.

If codes permit and a separate ground wire is used, it is
recommended that a qualified electrician determine that the
ground path is adequate.
Electrical Connection
To properly install your dryer, you must determine the type of
electrical connection you will be using and follow the instructions
provided for it here.

If local codes do not permit the connection of a neutral ground
wire to the neutral wire, see "Optional 3-wire connection"
section.

This dryer is manufactured ready to install with a 3-wire
electrical supply connection. The neutral ground wire is
permanently connected to the neutral conductor (white wire)
within the dryer. If the dryer is installed with a 4-wire electrical
supply connection, the neutral ground wire must be removed
from the external ground conductor screw (green screw), and
secured under the neutral terminal (center or white wire) of the
terminal block. When the neutral ground wire is secured under
the neutral terminal (center or white wire) of the terminal block,
the dryer cabinet is isolated from the neutral conductor.

A 4-wire power supply connection must be used when the
appliance is installed in a location where grounding through
the neutral conductor is prohibited. Grounding through the
neutral is prohibited for (1) new branch-circuit installations,
(2) mobile homes, (3) recreational vehicles, and (4) areas
where local codes prohibit grounding through the neutral
conductors.