You need an equaliser. Not any specific equaliser plugin, but any equaliser available in whatever application you choose to use. It may be a plugin, it may be built into the application.
If you genuinely only want to keep frequencies below some cutoff, then you want a low-pass filter. You'll find them in the list of EQ options, or you might find a separate plugin for it.
From reading your question though, this is not what you want to do. As piiperi's comment says, these higher frequencies are what make your guitar sound like a guitar. With just the fundamental frequency it would sound like a pure sine wave, because that's what it'd be. (A tuning fork is the closest physical equivalent of this.)
It may be that you've got stuff on there which you don't want though. If you've used a piezo pickup, they have a midrange "quack" to their sound which changes the tone. If you've used a microphone and you've not done a good job with positioning and/or playing, you may be picking up fretting noise, finger squeaks, poor picking/fingerpicking technique, clothing rustling, etc.. These are things you mostly don't want - although sometimes you do, because on most acoustic guitar playing you'll hear some of these things, so be wary about aiming for a completely clean, sterile result. But assuming you do want to remove them...
It's time for more EQ. You can use a multiband graphic EQ, traditionally with 31 faders for bands of frequency, or you can use a parametric EQ where you set a frequency yourself. Since you're a beginner, I'd suggest a graphic EQ is going to be easier for you.
You want to start with everything zeroed (at midpoint). Work out roughly where your troublesome frequencies are. Then one fader at a time, push that frequency up (at least halfway, to maximum if you can stand it) and run your recording. The result will sound strange, but you're not listening for sound quality, you're listening to see if this is a frequency band which is adding a ton of crap. If it is, then boosting that band will make the noise really jump out at you. If it is, push that band down to maybe -6dB, and then move on to the next one.
The idea is to have as few frequencies removed as possible. That leaves the tone of your guitar substantially intact, whilst doing a precision strike on the bits you don't want. Initially you're going to be far too heavy-handed, and the result with loads of bands removed is going to sound dull and muffled. No matter - go back and try again.
Doing good EQ is a skill, and it takes practise. A good live sound engineer can dial this in by ear on the mixing desk within seconds, because they know what they're listening for. It could take you half an hour to get something you like. No worries, you'll get there.
But any engineer can also tell you that the best way to get a good sound is to start with a good sound! Your guitar technique is part of that, of course - but after that, you need to record it well. This SoundOnSound article is a very good place to start for techniques on recording guitars. There are many ways to skin that particular cat, and it would certainly benefit you to do some experimentation with that. (And one thing to avoid - never, absolutely never, point the mic straight at the soundhole!)
In general, I would thoroughly recommend you going to the SoundOnSound site and looking through their archives. They've got a lot of articles about how to do recording and how to "massage" recordings with EQ and other FX. You'll learn a lot by going through their articles and then trying out some of those ideas for yourself.
