system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Let's say an Imperial or imperium-friendly person comes up with a reliable time-travelling device,

what would be their first temporal interference in order to assure Imperial survival and domination?

(Assuming Back to the Future-type rules for time travel without nonlinear shenanigans and paradoxes)

Edit: to make it more interesting, can only transfer your own body (and mind). No trans-temporal psychic tricks.
gpt
Probably the best strategy would be to pop back to when Standard Template Constructs were still readily available, lay their hands on several that history records as 'lost', and stash them somewhere where they will be safe and stable for several centuries so you can go back to your home time, dig them up, and hand them over to the Imperial authorities. This would be, in layman's terms, a Big Fucking Deal; the best the Imperium typically finds of the old STCs is old hard-copy printouts of the designs they made, which made 'lost' technology no longer lost. In the lore a couple of Imperial Guard mooks who found the STC blueprint for a better knife were literally put in charge of planets as a reward. All of the 'Land' vehicles? Named after the dude who found the blueprints. An intact STC could hypothetically be used to build *any* piece of technology from the peak of Humankind's knowledge and mechanical skill, and could represent a complete game changer in terms of the Imperium's capacity in all fields of endeavor.

Changing the timeline is a suckers game. Even in 40K, where enough facts are lost to the mists of time that you could probably make a big change w/o blowing up the timeline, it's difficult if not impossible to fully calculate the impact of changing the past in a meaningful way. But one can abuse the timeline without changing it.