John 14:15 reads in the NA27 (and NA28):
Ἐὰν ἀγαπᾶτέ με, τὰς ἐντολὰς τὰς ἐμὰς τηρήσετε
However, there is a manuscript discrepancy that would render the last word as τηρήσατε. According to Metzger in his Textual Commentary on the Greek New Testament (2nd ed.),
A majority of the Committee preferred the future tense τηρήσετε, read
by B L Ψ 1010 1071 1195* 2148 al (and perhaps supported indirectly
by witnesses that read the aorist subjunctive τηρήσητε, P66
א 060 33 al), instead of the imperative τηρήσατε, which, though
rather well supported (A D K W X Δ Θ Π f 1 f 13
28 565 700 892 Byz), accords less well with ἐρωτήσω in the following
In other words, both readings are fairly well supported. It appears that the first translation committee (1984) sided with the future active indicative reading (τηρήσετε), and the second committee (2011) went with the aorist active imperative reading (τηρήσατε).
It should also be noted that a (lesser-supported) variant also exists, the aorist active subjunctive (τηρήσητε). This could potentially render the verse something like the following:
If you love me, you would keep my commandments.
Concerning internal evidence, 14:15 begins with a subjunctive clause (Ἐὰν ἀγαπᾶτέ) which are often followed by future verbs, and Metzger mentions that the immediate context of 14:16 also uses a future active indicative verb (ἐρωτήσω, "I will ask"). This seems to support the future active indicative reading (τηρήσετε).
Given the internal and external evidence, the future active indicative reading (τηρήσετε) seems like the best option (the choice of the first NIV translation committee in 1984).