Can I fit a 39Wh battery in my MacBook Air 2011 which states 35Wh? I'm trying to change the battery in a MacBook Air 11″ i7 1.8 GHz (July 2011) MD214LL/A.

I found this Runpower battery, which is 39Wh.

Yet when checking the specs mentioned above, the battery is a 35Wh.

If I change the battery to one with a higher Wh, will it have a negative impact or will it be ok?
Can I safely buy and install the new battery?
The battery you want to buy should be fine (in terms of the specs and compatibility). The 39 watt hours basically means that on a full charge it should last longer than a 35 watt hour battery (about 11.4% longer). Many after market MacBook batteries do usually offer a little more capacity in that regard.

As for the battery itself, I've never used that particular brand, so cannot vouch for its quality. However, since it seems to have a 5 star rating, it looks promising. As long as it offers a decent warranty (I can't read French, so can't tell), I'd buy it.