Hello, I want to update MHW speedrun wich is totaly out, but when I click on submit, I can't change player name. But that"s not my run, how can I do that ? Thanks.

Only mods can submit other people's runs. Ask them to add the run.

ask them there and they might add it https://www.speedrun.com/mhw/forum

Yeah, there is absolutely no way that non-moderators could have the power to edit existing runs.

The vandalism would be unreal.