1. Gaining an additional action which is one space adjcent to the current action marker.
2. Adjusting the current action to one space adjcent without moving the marker.
Wihch one is correct? Gain one more action or just adjust?
The correct use is your 2nd option. Keep in mind that you can use multiple Flex tokens at a time, to take an action further away from the marker.
This is indicated by the following example in the book (emphasis mine):
For example, if Player 2 hits the number "7" but wishes instead to "Sweep!", located at "0", he would need to play 3 Flex Tokens.