It is absolutely safe.
However, I suggest the following:
a) Straight download the full firmware from http://www.iclarified.com/750/where-to-d...
b) Do a full backup of your phone in iTunes
c) Put your phone into DFU mode.
d) in iTunes, it will ask you to recover the phone.
In PC Hold Shift+click Recover button, or in Mac, hold Option + click Recover, and select the firwmare you downloaded in step a)
Once the recovery is done, select 'setup as a new phone'. When you finish stepping through the setup, Have iTunes restore the backup you made in step b).