It really depends on context.
If you delete a tuplet then its notes will spill to the right, overwriting following notes.
If you turn on Insert mode before deleting a tuplet then the following notes will be shunted to the right, which may cause problems downstream (particularly if you have multiple voices on the staff). In this situation it may be wise to cut and paste the material to a separate flow or an empty stave, alter the tuplets, then cut and paste back.
If you want to replace e.g. a 5:4 with a 5:3, select the tuplet (number or bracket) and hit delete. You should find that the first note that was previously in the tuplet is selected. Now type ; 5:3 Enter.
You can specify the note value of a tuplet explicitly within the popover, using w for whole note/semibreve, h for half/minim, q for quarter/crotchet, e for eighth/quaver, and then x y and z for 16ths/semiquavers, 32nds/demis and 64ths/hemis, e.g. 4:3e for fourth eighths/quavers in the time of three eighths/quavers.