Hi Jack,
Welcome to this Forum.
Let’s say you have a nice progression of your own. Cmaj - Fmaj - Cmaj - Gmaj.
You want to change it so something “more interesting”, like Cmaj - Gmin - Fmaj - Cmaj.
So, basically, you pass your MIDI, either directly from the keyboard by detecting it (the nice black button with a red dot) or detecting it from a MIDI file.
This will fill your progression it the section “A”:

BTW, Scaler will tell you that you played the C Major Scale (yeah, that’s right).
In the Section B, you’ll see the chords of C Major Scale:

You can drag or select (lasso) the chords in A, right-click and select to add these 4 chords to Section C:

There is another option to select:
Right-click in the free space left of the first chord or between any chord and then Select all chords:

You are in the section C? Here, you can do the magic.
If you want to preserve your original chords and add a few more to change, either in Selection A or in C, select all chords and add the to a new pattern.

If you only need to replace some chords, you don’t need to do this previous step.
Right-click on Fmaj that you want to modify and select Edit Chord in contextual menu:

This will lead you to a new page where you see your chords at the bottom and proposed chords in the middle. But you want to replace the current chord with something else.
First option: As you know you like Gmin, click on the Circle of Fifths (COF), on Gmin:

This will suggest you the chords from COF:

Drag Gmin from COF to your pattern and that’s it.
Go to another chord (Cmaj) and right-click to edit it. Do the same as previously by clicking Fmaj in COF.

Drag Fmaj to the pattern. Finish with selecting Cmaj in COF and dragging Cmaj over the Gmaj.
If you have no idea which chord to use instead of some others, do as follows.
Select the chord to change and go to Edit, as previously.
You’ll see that not only there is a Gmin (1) at the bottom, but also its fingering (2) near the top of the window
Click with your mouse on the key you want to disable e.g. note F and it will change the color to white.
Select G (also changes the color to pale brown.
By toggling the other keys, you’ll have Gmin in the end:

Or select it from COF 
When finished all the changes, click on Main:

and that’s it.
I hope this is clear. This was my first tutorial!
If you have other questions, don’t hesitate.