If two or more voltage sources $V_1$, $V_2$, ... $V_n$ are connected in series they can be replaced with a single voltage source with $$V_{eff}=\pm V_1 \pm V_2\pm...\pm V_n$$ where the terms in the right hand side are taken with $+$ sign if the corresponding voltage source $V_i$ is oriented in the same direction with $V_{eff}$ and with $-$ sign if $V_i$ is oriented in opposite direction with $V_{eff}$. Since the voltage sources are all connected in series, when we replace $V_i$ with $V_{eff}$, we need to replace the other voltage sources with short-circuits (wires).

For instance, considering the circuit in Fig. 1, voltage sources $V_1$ and $V_2$ are connected in series. Therefore, we can keep one the voltage sources, say $V_1$, replace its value with $V_{eff}=V_1-V_2$, and replace voltage source $V_2$ with a wire.

Voltage sources should never be combined in parallel.