Traveling back to 400 CE to introduce the moldboard plough and the three field system to Ireland and Great Britain so they can keep the fucking Anglo-Saxons out

Extremely blunt instrument method to prevent the evolution of the TERF

Traveling back to 1471 and equipping the Army of Novgorod with precisely three M1919 Brownings and a single Howitzer

Introducing attenuated Vaccinia virus inoculation with an extremely emphatic "no seriously, trust me, you are gonna need this" throughout the Americas beginning in the late 1400s, with routine visits to inoculate children through the 1700s

(The inoculation comes with an AK and a few dozen rounds for any tribe or village likely to encounter Europeans in the near future. If the Europeans are likely to be Spanish or Portuguese they get an extra set and a mortar just to be sure)