The thread on my Juki machine keeps breaking when I sew. How do i fix it. Also, How can i change the thread tension

There's a tension dial with numbers-0-9 --above the needle area- the smaller the number the looser the tension--the higher the number the tighter the tension

thread breaking-try this
try rethreading the machine
thread tension to tight
needle is bent or blunt
needle is inserted incorrectly
the thread is either too heavy or too fine for the needle

try this & see if it helps you
Cut 6-8 inch piece of thread of the spool you are going to use for your project
Take the needle you are going to use for the project
& insert the thread thru the eye of the needle
& lift one end of the thread to a 45 degree angle
& if the needle is the right size for the thread it will slide down the thread
But if the needle hangs on the thread you need one size larger needle for the thread