“How do I change my thoughts from negative to positive?”

A cognitive behavioral therapy approach can help. I would recommend a book called Mind Over Mood. If your negative thoughts are about suicide, I strongly recommend seeing a psychologist or visiting an emergency room.

To change your thoughts from negative to positive you have to start doing positive things. You got into the negative side by not doing and probably thinking too much about the past. I find that thinking about the past brings out depression and thinking about the future causes anxiety. The best thing to do is be in the present, but you need to empower yourself to make everyday a great day. Write a "to-do" list that will automatically bring you good thoughts, a good start is to do more for others and you will find that the joy you bring them will come back to you.

Some insight into your question can be found at:
https://thehealthsessions.com/changing-negative-thoughts/

Also:
https://www.psychologytoday.com/us/blog/what-mentally-strong-people-dont-do/201805/the-beginners-guide-changing-negative-thoughts