Changer une valeur de trigger uniquement pour un host

If that trigger is part of a Template, you can change the value on the template like this:

On configuration --> Triggers, choose Host Group = Templates, then in Host, select your template.
Next select the 'Too many processes' trigger [i.e click on Too many processes], change 300 to 400. Save.

Changing the value on the template will affect all hosts connected to that template.

If you only want to change the value for one host only, and that host is connected to a template, do this:

On configuration --> Triggers, choose your host. When you open the 'Too many processes' trigger it will not allow you change it. So, click on 'Clone'. Now you can change the value. Save.

Now you will have 2 x Too many processes, one with a value of 300 and one with 400. Disable the 300.