If your boot sector is full or is too much used, the main reason is the old installed kernel versions.
You can remove them if are not planning to go back using them.
Those 3 easy steps will help you to clean up boot sector in SSH:
(login as root first)
1. Check what kernel you’re using, and do not attempt to remove it:
2. Get list of all installed kernels
rpm -qa | grep -i kernel | sort
3. Remove old! only kernels
yum remove [KERNEL_NAME_AS_LISTED-EXCLUDING THE ACTIVE ONE]
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