Transpose guitar tab keeping the string number as possible ?

Select all the notes that need to be transposed (usually all the notes in the score, of course) in the tab, not in the standard notation if you have a linked pair of staves. Then use the Up or Down arrows to transpose chromatically up or down.

N.B. If one or more of your notes is played on an open string, obviously you cannot transpose down in this manner and keep it on the same string.

For example, a simple "melody" in C major:

Select the tab notes and press the Up arrow nine times (nine half steps from C up to A):

Notice that this does not change the key signature in the standard notation, but all the F's, C's, and G's are sharped. So simply add the correct key signature to the standard notation and it becomes what you would expect in A major.