To do it in situ you ned to use F&R whether you want to apply the replacement to many cells or just to one. The option to choose in both cases is
Current selection only.
To get the result in a different cell as a formula result or as an intermediary result of a subexpression in a formula use the SUBSTITUTE() function, or in less common cases the REPLACE() function. (What looks like a formula in your post doesn’t tell me much about what you actually want or need.)
(Edit 2018-09-27 00:25 CEST regarding the recent comments by the OP:)
A cell you just went to (single click, cursor keys) is not selected in the full sense. It only has the input focus what is shown by the applied cell-frame. If you Ctrl+click the same cell, it will be highlighted. Then it is also selected in the sense the ‘Current selection only’ of F&R is based on.
In addition: Your search expression and also the replacement contain special characters. Make sure that RegEx NOT is enabled.
Search in Formulae, on the other hand must be enabled.