R2, R3 and R4 are a delta resistor network. So is R3, R5 and R6 but we only need to transform one to an equivalent Y resistor network.

The formulas are here: https://en.wikipedia.org/wiki/Y-Δ_transform under Equations for the transformation from Δ to Y in the Basic Y-Δ transformation section. Plug in R2, R3, R4 for Ra, Rb and Rc. Once that’s converted to a Y you can do the usual series and parallel conversions.

The second answer here shows it visually: https://electronics.stackexchange.com/questions/447863/how-to-calculate-equivalent-resistance-in-mixed-circuit

It shows the steps to convert that H part to a single resistor. The only confusing bit is that Rab=R1, Rac=R2 and Rbc=R3 in his diagram.