Replace Sprinkler Tee Pipe

You can not, you need to replace it

Either the pressure has gone down due to a leak or the heads have gotten dirty inside and need to be cleaned out so they again pop out fully.

chemical pump malfunction or chemical run out.

There are several ways to adjust water pressure on sprinkler systems. First you have to determine if there is a problem with the sprinkler system itself. Things to look for:
1.) make sure that all valves including the main are all the way open.
2.) Inspect the sprinkler heads for clogging. Sediment and debris can also clog your sprinkler heads. Some debris removal may be required.
3.) You make have too many sprinklers operating by one valve. You may need to divide a zone and add another valve to increase water pressure.
4.) If all else fails. You can install a breather intake cap to allow air into your sprinkler system thus increasing GPM (gallon per minute)flow.
5.) one of the biggest mistakes that peopel make is that they undersize the water pipe size to save money. Typically you want to use 3/4" PVC pipe for your main runs and tee into 1/2" PVC for your sprinkler heads.
Good luck. I hope this helps.

I don't know what is your system set up. However, the system set up should be the following;
1. Your 1-1/2HP sprinkler pump must be installed at
a fix location nearest the water source. (Complete
with power and plumbing lines.)
2. At the end of the suction line is a vertical check
valve with strainer.
3. The suction pipeline must be connected to the
suction port of the pump.
4. At the discharge port, a flow control valve tee must
be installed. The flow control valve tee top is
covered with a plug, which serves as the priming
port when opened.
5. After the flow control valve, connect a short nipple
so that you can put a gate valve to stop the water
flow or allow the water to flow. You may connect
a pressure tank at this point or you may connect
the pressure hose going to the sprinkler.
6. To prime the pump, open the plug at the flow
control top. Fill it full with water, replace the
priming port plug.
7. Turn the flow control valve to full closed position.
8. From full close position, switch the power on.
9. The pump now run on and off due the pressure
switch setting. Slowly open the flow control valve
such that the water pressure remains steady.
10. Take note of the pressure.
11. Shut the power off, and connect the sprinkler
pump hose.
12. Put the power back on, and allow the pump to
run with the sprinkler.

Under this set up, you can run the sprinklers when your pump is running. Hope this will solve problem.