What is the mantra to be changed for Lord Murugan? I have health issues so I was advised to worship him. What should I chant for Lord Subramanyeswara?

 You may read and follow the post by Uddesya Sharma.

You may recite - Om Sa Ra Va Na Ba Va - 125000 times. Decide yourselves the numer of rounds of 108 times per day (say 90 or 60 or 30 rounds); no of days(say 15 or 30); same time -morning or afternoon or evening or night; Same Place. Take bath before reciting, light a lamp, incense. Have some fruits after.

If it is convenient, you may visit a temple called Kalugumalai in Tamil Nadu. It is recorded that this God bless healthy body. (Those who seek wealth are not recommended)

If it is not possible to visit the temple for you - I am enclsoing his picture for your worship while chanting his mantra.