Press it again with the input field selected and it will do just that -- the cursor will move, the text won't be selected anymore. Try it in your browser: click on the address field above. See how it selects the whole thing? Now click again.
If you want it to not select anything and simply position the cursor, have a look inside UIInput's Update() function, around line 590. Selection start and end are chosen there. Now examine UIInput's OnPress() function and note how if it's already selected, it will set both selectionStart and selectionEnd to GetCharUnderMouse().
