How to apply a low-pass or high-pass filter to an array in Matlab?

Look at the filter function.

If you just need a 1-pole low-pass filter, it's

xfilt = filter(a, [1 a-1], x);

where a = T/τ, T = the time between samples, and τ (tau) is the filter time constant.

Here's the corresponding high-pass filter:

xfilt = filter([1-a a-1],[1 a-1], x);

If you need to design a filter, and have a license for the Signal Processing Toolbox, there's a bunch of functions, look at fvtool and fdatool.

You can design a lowpass Butterworth filter in runtime, using butter() function, and then apply that to the signal.

fc = 300; % Cut off frequency
fs = 1000; % Sampling rate

[b,a] = butter(6,fc/(fs/2)); % Butterworth filter of order 6
x = filter(b,a,signal); % Will be the filtered signal

Highpass and bandpass filters are also possible with this method. See https://www.mathworks.com/help/signal/ref/butter.html