Today i was doing some experiment on my device. i found a way to change system font in xperia m.
Root file explorer like es file manager or root explorer
1.Download any fonts which you like
2.We need two font files, one for BOLD and another one for REGULAR.So if you download two different fonts for BOLD and REGULAR, that's fine.or if you need same font for both bold and regular, copy the downloaded font and paste it there(so it will be like "copy of *****.ttf)
3.Rename one file as "SoMABold" without quotes (if you downloaded different BOLD font then rename that BOLD file)
4.Rename other file as "SoMARegular" without quotes(if you downloaded different REGULAR font then rename that REGULAR file)
5.Now go to system/fonts and paste both files (SoMABold and SoMARegular) and set permission as rw- r-- r--
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