How to Change MTU

Can someone please tell me how to change the MTU from 9000  to 1500?

If I understand your question correctly, you are willing to change the MTU on you network adapter. Not sure what OS are you using, but if you are on Linux, you can change it like:

1) List the available adapters and choose the correct one
"ip link list" or "ifconfig"

2) Let’s imagine your network device is “eth0” so if you need to change the MTU on it, you can execute:
"ip link set dev eth0 mtu 1500"
"ifconfig eth0 mtu 1500"

3) To validate your changes are applied, execute again:
“ip link list” or "ifconfig"

I hope you find this information helpful.

I must be doing something wrong; for some reasons, I can’t set the MTU. Here’s the screen

The error message is clear, that you are using non-existent interface/device. I used “eth0” only as example, you should change that part in your commands with the interface that you are having (“ens5” in you screenshot). The whole point of executing “ip link list” is to see the list of available adapters, and choose the one according to you case/needs.

Sorry, I really didn’t know about that command. But  changed the MTU in that interface and it works now. This issue can thus be considered closed.