You need to be more specific in your question. In what way do you need to change instrument? Did you previously write a score for trumpet and now you want to write a score for trombone? Or do you have a score with many instruments, for instance a trupet part, and you want the alto sax player to put down his/her instrument and pick up a clarinet in the middle of the score?
Otherwise, the obvious answer is:
1. put down your trumpet
2. pick up your trombone.
3. start playing.
Comments
You need to be more specific in your question. In what way do you need to change instrument? Did you previously write a score for trumpet and now you want to write a score for trombone? Or do you have a score with many instruments, for instance a trupet part, and you want the alto sax player to put down his/her instrument and pick up a clarinet in the middle of the score?
Otherwise, the obvious answer is:
1. put down your trumpet
2. pick up your trombone.
3. start playing.