My shower pressure in my upstairs bathroom is fantastic. However, the same cannot be said for the downstairs bathroom (ground floor). Given the fact that upstairs is disadvantaged, this does not make sense unless there is a pump serving that bathroom alone which i don't think there is. So i want my downstairs shower to reflect the upstairs one. I'm not entirely sure but i think the question is how do i decrease the pressure in order to increase water velocity. Or how to I increase the flow to increase velocity. My hot water is supplied by a gas boiler.
Your downstairs shower has a restriction of some sort in the water supply. Start at the showerhead and work backwards to find it. You need to check pressure while flow is happening - a restriction will allow full pressure when there's no flow, but will cause a large drop in pressure when there is flow.
For the easy possibilities, check any supply valves to be sure they are fully open, and check that you don't have a limed-up or otherwise overly restrictive showerhead. Other issues can be much more involved to find/resolve - my classic warhorse of a tale is the shower/tub with terrible flow on the hot water side, due (found after I reached the point of disassembling pipes, because something was clearly very wrong with them) to one elbow being 95% full of solder, leaving only a tiny hole for the water to pass through.