I have a Metz Mecablitz 44 AF-1 digital for canon, so I can only use it in manual with my nikon d7100. If I change its firmware to the nikon one will it work with nikon or just break?
You can use it as a manual flash on the Nikon hotshoe without doing anything to it.
I wouldn't recommend trying to upgrade the firmware. The flash is physically only capable of being a manual-only flash on a Nikon camera. Look at the pin layout on the foot vs. the Nikon hotshoe. Canon's pin/contact layout has four small contacts in a square underneath the sync pin. Nikon's pin/contact layout has one pin above the sync pin, and two below--placed where the Canon pins/contacts won't touch them.
Those pins and contacts have to touch for the camera to communicate with the flash. The sync pin will always be placed identically on any ISO-compatible hotshoe. But the other pins are where the TTL, HSS/FP, 2nd curtain, zoom, etc. communication happens. Assuming that's why you want a Nikon-version flash, understand that simply can't happen physically. In addition to this, some of the signals are in reverse voltage: for example, the quench pin signal on Nikon goes the opposite direction of the quench signal on the Canon foot. This isn't necessarily something a change of firmware can switch--it requires different wiring.
You're going to have to purchase a Nikon-dedicated version if you want full function on a Nikon hotshoe.