Welcome to the forum!

The basic steps are:

1. enable reception of Bank Select and PC (program change) messages on the instrument

2. send the appropriate MSB, LSB and PC messages (in that order) from the external device to the instrument

The MSB/LSB for presets are listed in the Data List doc for each instrument and instructions for enabling/checking bank select and PC reception are in the reference manual for each instrument.

Yamaha makes all docs available online - https://usa.yamaha.com/support/manuals/index.html

When you post please provide as much detail as possible including: instrument model and OS version, any presets being used, any cabling being used including cable types.

If it was working before then either the iPad is doing something different (which I doubt) or the cable between is failing (which is possible, but ...) or the MODX (assumed due to the forum category) of some flavor ,(not stated - M or not M) has changed.

The MODX can be configured to ignore bank and PC messages.  Make sure this isn't configured.  There are both global MIDI ignore and per Performance settings that can both ignore these messages.

The MODX can be configured to not use USB so it may not be communicating with the iPad at all.

These messages are channel messages so the MIDI channel sent from the iPad needs to match the MIDI I/O channel on MODX.  This can be configurable and this is where the MIDI implementations between the M and non M models start to make a difference in how each respective device can be configured.  Generically I can say that MIDI settings which change the listening channel for these PC/MSB/LSB messages may have changed.  Most of these settings are per Performance although there are also some global settings which, on the non M (classic) MODX for example can alter the target MIDI channel.  Specifically for these non M models it would be multi vs single channel MIDI modes (or hybrid).  

So ... There's somewhat of a checklist to run through.