I’m Dyslexic and took years and years AND YEARS! of music theory and advanced theory and then had to make in work in my head: The Keep It Simple Stupid personified!
I play semi-pro now but Pro for 15 years and have taught music.
THE BRODIE has a great idea with the Circle Of Fifths, but if it doesn’t make sense, try this: Put the chords of the song on paper, is a straight line. Decide what Key you want to experiment with and count up, making the original Key #1 – for instance; the song is play in E but your singer needs really tight underwear to do it now, so you want to go to G (I’m in this boat now, can you tell?)
E F G is 3 notes (E=1 F=2 G=3 sharps and flats don’t count, yet)
Your “major” chords are 1, 4, 5 and possibly the minor, so
1 4 5 6minor
E A B Cm becomes
G C D Em
If you know Barre Chord patterns, you will notice they won’t change much. I go down in half-steps from the highest note in my limited singing range, so move down one fret at a time until your vocals sound top-notch again. Sometimes, just changing the voicing of the accompaniments can help. Explore that too.