Depends on your focus. If you want to go somewhere and compete at a higher elevation than where you live and ride, then there are acclimatisation systems that can help train your lungs to work better on lower pressure and lower oxygen levels.
The highest cyclable road in the world is 19,005 feet/5792 metres elevation, and is in Bolivia. The top 10 are in Bolivia, Tibet, India, so in the Andes and the Himalayan mountains, and all are over 17,500 feet/5334 metres.
At those elevations there is less oxygen available, so your lungs have to work harder to keep your blood/oxygen ratio higher.
Altitude (m) Effective Oxygen % Altitude Category Example
0 20.9 Low Sea Level
500 19.6 Low
1000 18.4 Medium
1500 17.3 Medium Boulder, CO
2000 16.3 Medium
2500 15.3 High Aspen, CO
3000 14.4 High
3500 13.5 High
4000 12.7 Very High
4500 11.9 Very High Pikes Peak
5000 11.2 Very High Mont Blanc
5500 10.5 Extreme
6000 9.9 Extreme Kilimanjaro
6500 9.3 Extreme
7000 8.7 Extreme Aconcagua
7500 8.2 Extreme
8000 7.7 Ultra
8500 7.2 Ultra
9000 6.8 Ultra Mt Everest
So you only have to go up 500 metres to for O2 to drop by 1.3 percent of the available air, which is an effective drop of 5% of the available oxygen.
At 5,500 metres the available oxygen is half what you'd have at sea level.
What to do?
Null case - Nothing If you're just tootling around, you don't have to do anything. Simply be aware you may suffer from shortness of breath, dizziness, headaches, nausea and vomiting, brownouts, blackouts and fainting, so rest periodically and don't push through any warning signs.
Here's an example of a purpose-built facility for altitude training. The room has spin bikes and the pressure and oxygen ratio is controlled while you ride. Plus you're monitored by a human, and low blood/oxygen levels will be avoided.
There could be something similar closer to you.
There are Blood Oxygen booster drugs like Acetazolamide aka Diamox which is used to treat Altitude Sickness. They're treating the symptoms after onset, not preventing the problem in the first place.
Your body will acclimate to the new condition, given enough time. However hanging around at height for a few months is an expensive waste of a holiday. So you can use Altitude training, either by riding a trainer in a room with oxygen level controls, or sleeping in a mask/tent with an artificially low oxygen level.
Here's a tent to sleep in for 2 months before your event/trip. Downside, not cheap! And probably really annoying.