Short answer: don't think you can. You may be out of luck if you bought secondhand or acquired.
For a while, a few years ago, NowTV streaming service in the UK resold Roku devices under their own name (and mostly white/yellow, late black/light blue, rather than black/purple). The very oldest of these are desupported and don't function at all, and BBC iPlayer has also desupported. IDK the specific Roku type model numbers (the trailing channel letters seem to be SK=Sky), but low end boxes and 2 gens of stick were available.
NowTV also whitelabled Roku account website and channel store but this is completely separate from Roku account/UK channel store. NowTV and Roku won't recognise other brand of device during setups (something that Roku could choose to fix, put in a suggestion).
The remotes are compatible across brands, I think
As of May 2021 in UK, NowTV no longer sells branded Roku, unsure if NowTV account for streaming device/channel store is still available. (Update:seems to be available). Full service still available in Ireland. Any boxes/sticks for sale are used or "new old stock" or Ireland market stock.
(Background: NowTV UK, currently styled as "Now", unrelated to any similar named streaming app in US/NA, is the "budget" brand of Sky Satellite/streaming service, now owned by Comcast. Sky is also a big ISP in the UK, LLU phone line ADSL2+/VDSL2 available in most UK phone exhanges, some FTTP/FTTH, and Now also resells that as NowBroadband. Sky is focussing streaming device investment on promoting their new own brand SkyGlass TV to compete with RokuTV and other streaming TV and devices..I think it is based on AndroidTV)
I don't have NowTV device/online account, so I can't fully check some of what I have put above. I do use Now streaming service on Roku and have NowBroadband, but that's unrelated to streaming device.