When you are woodshedding and learning how to play a new piece, sometimes you have to change it to make it work for you.
How it sometimes goes for me is, I remember a song I liked, or hear one I haven't heard in a while, and go looking for the the music. Mostly I prefer tablature notation and chords, since reading standard musical notation, with the notes and staves and all, is something I do poorly. Too lazy to practice.
Sometimes, I can work it out on my own, since, if the chords are fairly simple and I can hear them well enough in the piece, I can fake it.
If you are sans band, and you play the guitar and sing along, probably you sing the melody and strum or fingerpick chords, i.e. the rhythm; least that's how it works for me.
So, I get the music -- sometimes in a book, sometimes by itself, and discover that the song I want to cover is in a key that won't let me reach the high or the low notes. I'm a Beatle fan, do a half a dozen of their songs, and they had a vocal range at the top like chipmunks, shading into the ultrasonic. I can't get there from here.
So, for example, "Blackbird," if I am trying to sing it in the key of G-major, which is what the mop-tops sang it in, when I get to the end of the first line -- /Blackbird singin' in the dead of niiiight/ -- I'm not going to get "night" unless I drop my voice an octave, or raise it into falsetto. Both work, but neither give me the tone I want.
So, how to fix that?
You can use a capo -- a spring-clamp that alters the key by raising the pitch across the six strings. (Guitars strings are tuned relative to each other, and the standard tuning is, from low to high, E-A-D-G-B-E.) Every fret you go up, you change either a half or full step.( On Harrison's "Here Comes the Sun," he capos the guitar at the seventh fret and plays D-shapes. Since I play that one fingerstyle and don't sing, it doesn't matter which key it is in, and I play it wherever I feel like. Sometimes up and down the neck, sometimes with a capo.)
You can change the key by playing higher up the neck and using barre chords. (Generally -- not always -- the first chord you play in a song tends to be the key it is in.)
An E-chord shape on the first fret becomes a G-chord played on the third fret and barred on the second. Once you know a few shapes, you can slide them up and down the fretboard and change keys, simply by where you start.
You can retune the guitar higher or lower. In my case, I prefer to tune a full step down, so that the strings are D-G-C-F-A-D. Playing "Blackbird" in G-major shapes thus becomes the key of F-major, and with that one, I can manage the high note on "night." Mostly.
You can also change the key by changing the first-position chords -- aka the cowboy chords. Since the tuning is relative, if you have a song in, say, A-major, and you want to change it to C-major, you just shift all the chords in the song over two. A becomes C; B becomes D; C goes to E, and so on. This works pretty well, until you start getting into sharps and flats, and that's beyond me to try and explain.
In chords, -- at least two notes, mostly three, sometimes four or more -- there are also minors, suspended, dominants, 7th's, 9ths, and a bunch of others. Jazz players have some fingerbreakers whose names are really complex. 7th-flat-sus-13th-ninth-with-a-side-of-fries ...
To change a song I can't sing into one I can, I've done all of these at one time or another. It depends on the sound you want, and sometimes playing up the neck sounds better than changing the first-position chords. You noodle around and use your ears to determine which sound you like best.