The ACR Plugin does not get installed into Lightroom. Lightroom is a standalone application and each version has the equivalent of a specific ACR version. e.g LR 6.x has the equivalent of ACR 9.x, the latest update to LR is 6.6.1 and this has the equivalent of ACR 9.6.1.
You will need to upgrade to LR 6 if you wish to achieve this.
You have 2 options.
1) Upgrade to the newest version of LR. That is either the stand alone version 6 or the subscription based Photography plan that includes both LR and PS.
2) Use the newest version of the free Adobe DNG converter program, 9.6.1, to convert the Fuji X-T2 RAF raw image files to DNG files and then LR 5 can read them.
As for ACR LR has that built in and it can't be added or updated/upgraded by it self. So the only way to get the newest version of ACR in LR is to install the newest version of LR.