In the game you find the setting under Settings → Mouse/Controller → Mouse Sensitivity.
Conversion
Rainbow Six Siege to Apex Legends Sensitivity
In short: Rainbow Six Siege sens times 0.2604. 10 becomes 2.6. For your own value, just edit the sentence below.
How to set it in Apex Legends
What changes when you go from Rainbow Six Siege to Apex Legends
Apex Legends turns 3.84 times further per mouse count than Rainbow Six Siege. That is why the number gets smaller while your mouse distance stays the same.
Apex Legends shows 14° more field of view than Rainbow Six Siege by default (105.5° against 91.49°). The picture looks a little wider and movement on screen a little slower. For Apex Legends the field of view is only a rough guide, check it in the game.
In Apex Legends you can enter up to 2 decimal places. Use all of them to keep the conversion exact.
Common questions
How do I convert Rainbow Six Siege sens to Apex Legends?
Multiply your Rainbow Six Siege sens by 0.2604. 10 in Rainbow Six Siege becomes 2.6 in Apex Legends.
Where do I set the sens in Apex Legends?
Settings → Mouse/Controller → Mouse Sensitivity.
Why does Apex Legends still feel a bit different?
Rainbow Six Siege: Slider from 60 to 90, vertical (default 60, about 91° horizontal at 16:9). Apex Legends: Slider from 70 to 110 (about 105 to 106° horizontal at 90). Match the field of view as closely as you can; then the converted sens feels most natural.
Does this also apply to scopes?
No. The values are for hipfire. Zoom and scope sensitivities are set separately in the game.
Should I change my DPI?
No. Keep your DPI and only change the sens in the game. That keeps your feel the same everywhere.