How do you change the mainspring on an air arms jackal?

Your first problem is locating a schematic for the rifle. I suggest you contact Air-Arms at the link below and ask if they could send you one. The next problem is locating a Spring compressor in order to install the new spring. (Not Easy) Spring compressors are not easy to locate.

you may want to have a Air gun shop do the repairs for you. I would look up "air gun repair" on the web and find someone near you to repair the rifle. Or see the #2 link below.