To open the Editor, tap on the pencil in song view
https://technimo.helpshift.com/hc/en/3-ireal-pro/faq/245-editor-s-buttons/?p=android
There are two ways to input or edit chords in the Editor:
1. Type the chord manually via the keyboard
2. Touch the letter name of the chord, then either use the keyboard to enter the rest of the chord, or touch the 3 dots to the right of the chord entry box, swipe and tap the chord quality to be added.

Old version (iReal Pro v2021.6 or lower)
To open the Editor, tap on the pencil in song view or hold on song title in Song index for menu -> Edit.
https://technimo.helpshift.com/hc/en/3-ireal-pro/faq/354-old-editor-s-buttons/?p=android
There are two ways to input or edit chords in the Editor:
1. Type the chord manually via the keyboard that appears when inputing in the text field, then tapping Enter
2. Using the C7 button next to the text field to first choose the root,then tapping again, to display a list of available qualities (only possible if you have already chosen a root).