Ask more questions
Basically, don't jump to conclusions. You can't form a proper understanding of a person without having all the facts. Say, you're talking to a potential friend and that person tells you they don't drink. Your first thought might be that this is a boring person or a person who is very wholesome. But, there's not a lot of evidence to back that opinion up either way, yet. You have to really get to know a person to understand their full personality and you might miss out on having a great person in your life, if you make a quick judgement.
This isn't just about rejecting someone either, if you form your understanding on someone through judgements, instead of observations, you can't truly know that person on a deeper level.
SELF-CARE SHOP
Look at your own behaviour as well as the other person's
People have a tendency to act differently around different people. If someone's engaging in a behaviour that you don't like, before judging their behaviour, check if the way you're acting is causing that behaviour within the other person. Sometimes, it really does have nothing to do with you, but sometimes it does. Judging another person for being argumentative, when you're always starting arguments yourself, gets you nowhere.
Try to have a open mind
If you enter into new situations with prejudice, you can miss seeing a situation for what it really is. This often happens when you've been hurt a few times and you want to stop yourself getting hurt again. If you've been hurt by a few guys with red hair, you might be tempted to think all guys with red hair are bad or all men are bad. These thoughts are made on feelings rather than facts. It also translates into everyday life too, like presuming all women like to talk about their feelings. In reality, stereotypes don't always hold true.
Look at why you're being judgemental
If you're a person who is constantly judging others or yourself, you might want to reflect on why you're doing that. Only you can answer that question because we all have our own reasons. In the past, I've started to judge people when they've disappeared on me whilst dating. It's happened because I've found the uncertainty anxiety inducing. Understanding why you do things, helps you to reign yourself in and stop you hurting yourself or those around you.
Get to know or observe lots of different types of people
This can help you stop prejudice, once you get to know a variety of different people, you can understand that your original negative thoughts, about a group of people, don't always hold true.