If you want to remove rows with Time == T0 and Value > 5, I think this code does what you want.
DF <- data.frame(Subject = c("A1", "B0", "A3", "B1", "B3"),
Treatment = c("Amutant", "control", "Bmutant", "control", "control"),
Time = c("T0", "T0", "T3", "T1", "T3"),
Value = c(5.3, 4.8, 4, 3, 6))
DF
#> Subject Treatment Time Value
#> 1 A1 Amutant T0 5.3
#> 2 B0 control T0 4.8
#> 3 A3 Bmutant T3 4.0
#> 4 B1 control T1 3.0
#> 5 B3 control T3 6.0
library(dplyr)
DF <- DF %>% filter(Time != "T0" | Value <= 5)
DF
#> Subject Treatment Time Value
#> 1 B0 control T0 4.8
#> 2 A3 Bmutant T3 4.0
#> 3 B1 control T1 3.0
#> 4 B3 control T3 6.0
Created on 2020-02-12 by the reprex package (v0.2.1)