There's no actual way to fix UR, nor can you get low UR just by wishing really hard for it or even trying really hard.
It's affected by a lot of factors, and usually you will get waaay better URs on maps that are both very easy to tap, read and aim for you.
So generally any metronome map (maps that only feature 1 type of note at best, with maybe a triple here and there, usually pp maps) that is below your skill level and a decent OD will give you good UR results.
You should never expect to get good UR on a song that is too draining on your stamina, too hard to read, too fast, too hard to tap, and to a certain extent even too hard to aim for you.
You can't expect to magically get good UR on 270BPM maps when you just don't have the speed for them, and naturally your UR will turn bad when you misread notes, or if a song requires very good fingercontrol, as any kind of mashing (whatever the reason for it) will ultimately ruin the UR, even if it only happens a couple of time over a map, it does affect the median by a lot.
Also, you'll usually get better UR on maps with a very high OD, unless of course it comes with the difficulties mentioned above. Atleast for me it works like that.
I seem to subconsciously adjust to the looser OD on some maps and abuse it, while OD9-10 nets me better UR on average, as long as I meet all the other conditions required to play it.
An example for a map that makes it very easy to get good UR on is "Cold Green Eyes", as it's not hard to read, IIRC only has 1/2 notes and has a very comfortable BPM and no stamina requirement.
Pretty much any pp map without streams makes it easy to get good UR, even for beginners. Try "Cold green eyes" or "Get Jinxed".
TL;DR is: You can't improve UR. It comes naturally and is more of an indicator of a certain degree of mastery over a map. Even players with usually a very good UR will struggle to achieve the same UR on a map that is way more difficult (for whatever reason) than the ones they got their UR record on.