A sharp key signature would allow you to use bar chords, or alternate chords, or a capo. But, a flat key signature requires you to tune down in order to match the songs key. Ukantor suggested "Don't re-tune your uke. Just play the appropriate chord "shapes""...but, if the key is lower than the open strings I must tune down.
Thanks again everyone!! I have a lot to learn.
Yes of course you have some to learn. You can find all the chords in sites like this, my fave:
https://ukebuddy.com/ukulele-chords
And play like my 2. option or ukantor's suggestion without retuning any or using a capo.
Key signatures don't imply that some key is higher or lower exactly.
My option 3. is a one example of that you can use always use a capo if you know chords in some other "lower" key, like G in this case. If you only know C key chords, capo won't work for you with Bb.
I never use a capo with uke, but it can be a quick save, when not knowing a key's like Bb chords fingerings.
Circle of fifths is a good one to find all your 6 basic chords in any key. In C/Am key they are all in that 11, 12 and 1 (13) hour sector. F, C and G(7), Dm, Am and Em (sometimes E(7).
https://en.wikipedia.org/wiki/Circle_of_fifths
Just the circle picture,for heavens sake ignore the words
Lets think about keys of C, F and G. If some song is not suiting your singing range in C, then most likely F or G will work. F and G are close as root notes, even if other has one flat as key signature and the other one sharp.
But there is a twist. When C key does not suit your singing range, singing in G you can either sing a fifth higher or a fourth lower. Some "duality". Or in Bb you can sing a whole tone lower or minor 7th higher than in C.
That circle is a powerful non mechanical transposing tool too.