Hi Jacena
one option is to use the usb cable that came with the phone Connect HTC Desire C to your computer, and select the Disk drive mode and done (pops up on phone screen - ensure the phone is unlocked).
on the pc your drive will show up under My Computer as HTC Storage (95.2mb)
double click the icon then you will get a list of folders select mobile, double clicking to open and you should have two folders gallery and music
open the music folder then you can delete or add by dragging your music to the empty folder (do same for photos in gallery)
close the folders back to disk icon right click and select eject to remove the folder from your system
allow phone time to update music/pictures
If you use Astro select select Manage My Files should find yourself in /mnt/sdcard
select up Icon, now you should be in /mnt on the screen you should now see a folder emmc select this
now you should see a folder mobile select this now you should have 2 folders Music and Gallery
Select Music and delete to your hearts content by holdind on the album folder or the title up pops a list 4th one down delete
hope this helps