most common...
you just ftp the new buttons to overwright the old buttons..
example the post reply button here on vb.org is
Code:
https://vborg.vbsupport.ru/images/cstyle/blue/buttons/reply.gif
so if i wanted to change that button i would name the new replacement button "reply.gif" and upload it in the buttons folder.. somtimes you have to clear cookies to reflect the changes. most button sets come in a pack so you can name the folder to buttons and upload them all at once or you can repeat above step and do one at a time if your not changing them all.
alternate way (if you wanna save your current buttons and not overwright them) is upload your new buttons in a folder on your sever and go to your style manager in admin cp and change the "path to buttons" to reflect new location. caution is most button packs dont include all the images in the button directory. so if you create a new directory make sure all the images in button folder are presant.
right click on your current buttons and find the path or folder they are currently located in and replace them is the easiest way.
most common is images/buttons depending on how you set your forum up. another suggestion is download (ftp) your buttons folder to your pc. then drag and drop your new buttons in that folder on your pc to overwright old. then when your all done reupload that folder on your server. that way you can visauly see what buttons have been replaced and your sure you have all the misc buttons that your going to reuse.